Fear of the unimaginable in the wake of the spectacular collapse of the Twin Towers and the gouging of the Pentagon has been supplanted by fear of the believable: the target could be an airport from which people travel; the club where they catch up with friends; the shopping centre at which they get groceries.

The greater sophistication of today's social media magnifies talk of a dirty bomb to be as powerful as a dirty bomb itself – which makes you wonder why former US secretary for homeland security Tom Ridge would undermine his recent claim at a conference in the US, that America is safer than in 2001 – before listing a dirty bomb attack and poor nuclear security in the US as threats that worry him most.

Others are more sanguine. Marking the 14th anniversary of 9/11 in 2015, Richard Dearlove, who headed Britain's MI6 at the time of the attacks, observed drily that there had been no dirty bomb, no biological attack, adding: "It's quite easy to exaggerate the threat and overplay it."

The data suggests that Americans, in particular, are safer. But as analysed by John Mueller and Mark Stewart in Foreign Affairs, opinion polls suggest a "long-term, routinised, mass anxiety" since 9/11 "that shows little sign of waning".

The audacity of 9/11 transfixed the world – years of meticulous central control and transnational planning that culminated with hijacked commercial aircraft serving as bombs that killed about 3000 people in two iconic cities, in the name of al-Qaeda. Fast-forward to 2016 – and there's global whiplash in July, when two men enter a church in working-class St Etienne-du-Rouvray, in Normandy, France, and slit the throat of an 85-year-old priest as he finished Mass, in the name of Islamic State.

Mueller and Stewart write: "Over 70 per cent of those polled in 2001 believed that 'another terrorist attack causing large numbers of Americans to be lost' was likely. The figure was roughly the same just before the rise of IS. Along the way it was temporarily pushed up by some 10 percentage points by the London attacks of 2005, and a similar rise occurred after the recent [2015] Paris attacks."

Observing higher percentages believing in the likelihood of another major attack as far removed from 9/11 as in 2013 and 2014, they say: "Increased spending on domestic homeland security since 9/11 has totalled well over $US1 trillion, and efforts to chase down and eliminate terrorists abroad have cost trillions more – [but] these extraordinary expenditures have utterly failed to make people feel safer."

The nature of the terrorist threat is very much changed by upheaval in the world of terror. Osama bin Laden is dead and al-Qaeda is like a corporate dinosaur that lives on past glory, while the upstart IS operates with all the agility and brinksmanship of a Silicon Valley start-up.

Bin Laden hankered for a caliphate, but figured it best to wait until he had fought the "far enemy" – the US – and his recruits were mostly from the Muslim world. In the face of massive, US-led counter-terrorism operations in the wake of 9/11, al-Qaeda's leadership were pinned down in the borderlands of Afghanistan and Pakistan, and its satellites effectively became their own masters in diffuse, overlapping international networks but often focused on domestic, not global conflicts, and on seizing territory locally – see Yemen and Libya.

IS, by contrast, carved out a caliphate just as soon as it could – a now-shrinking swathe of territory that straddles the Syria-Iraq border. As a long-term venture the caliphate was crazy; but in the short-term is was an organisational masterstroke, serving as a haven in the midst of war, with a taxable population of millions, revenue-producing resources like oil fields and substantial arms caches to be plundered – from the US-supplied Iraqi government forces in particular.

The caliphate was a magnet for tens of thousands of foreign jihadists, with possibly hundreds having survived combat and being trained operationally before returning to their countries of citizenship in the West – to bide their time until they get the urge to strike unilaterally or on orders from the caliphate.

IS is less bound up with distinctions between "far" and "near" enemies. And while al-Qaeda bogs down on issues such as chains of command, IS is more laid-back, urging followers to strike wherever and however they might, using guns and machetes as weapons – or a truck, as in the case of the July 2016 attack in Nice, in which the vehicle was used as a battering ram to kill 86 and wound 434 in a Bastille Day crowd.

IS's orchestrated terror plots are inventive, usually involving multiple points of attack in cities in Europe or in the Middle East – but they never pack the punch that Bin Laden delivered in New York and Washington.

Many IS attacks are so-called "lone wolf" operations, in which investigators find no provable link between self-radicalised attackers and IS – like the husband-and-wife team behind the San Bernardino attack, in which 14 people were gunned down in California in December 2015; and the Orlando massacre, in which 49 were killed in June 2016.

Security experts concede that airport security is so intense and cumbersome that 9/11-style attacks are unlikely to be repeated – but airport security lines remain hellish. In the meantime, the experts admit the obvious – it's virtually impossible to thwart walk-in or drive-by attackers, as happened in San Bernardino, Nice and Orlando.

Terrorist attacks have become ridiculously cheap. Analysts estimate that the entire 9/11 operation cost al-Qaeda about $US500,000. More recently, its affiliate al-Qaeda in the Arabian Peninsula claimed its notorious "bomb-in-ink-cartridges" operation, which was uncovered before the devices were detonated on flights between Yemen and the US, cost a mere $US4200.

More recent attacks have been even cheaper – Nice cost only as much as the hire of a 19-tonne truck. Orlando was just the price of a few firearms. The Boston marathon bomb – it cost maybe $US100 for a device that inflicted personal and property damage in the vicinity of $US350 million.

IS also runs a more sophisticated propaganda machine – and its social media operation is remarkable in luring fighters and jihadist brides with unprecedented ease across linguistic, cultural and geographical borders. It encourages attacks in the West and reports the IS take on the news of its war. Its facility with the internet, for organising operations and grooming would-be attackers makes IS operations more difficult to detect than in the old days, when face-to-face meetings were the only option to connect.

Paying the movement a compliment, analysts Daveed Gartenstein-Ross and Nathaniel Barr write in Foreign Affairs: "[Islamic State] has capitalised on evolving communications technologies, building cohesive online communities that foster a sense of 'remote intimacy' and thus facilitate radicalisation … established a team of 'virtual planners' who use the internet to identify recruits, and to coordinate and direct attacks, often without meeting the perpetrator in person."

But there's something more here. Making the point that not all believers of the Salafist ideology, to which IS cleaves, are violent and that violence was not the sole drawcard for recruits, Jacob Olidort, who is an adjunct professor at George Washington University, argues there has to be something else – even if that "something" sounds a bit absurd.

"It is, rather, [Islamic State's] ability to sell and validate its world view in light of distinct circumstances that Muslim communities either experience or observe," he writes. "Specifically, for both those socially and economically disenfranchised by life in the developed world, as well as for those experiencing or witnessing the violent unrest in Syria, [IS] offers the promise of a tranquil and authentic Islamic state, full of opportunity for those who accept its authority."

The organisational depth revealed in IS' Brussels and Paris attacks is a pragmatic acknowledgement of military reality – with their turf shrinking dramatically in Iraq and Syria, IS needs to be embedded outside the region, in the midst of easy targets and immigrant communities in which their fighters have family and friends, if it is to continue its fight with the West.

That shift is the cause of great alarm, especially in Europe. Martin Chulov, an expert on IS, writes in The Guardian: "Little about what comes next is clear. The group's loss of territory has shifted geopolitical ground in ways that could not have been predicted – and in the eyes of many European governments, its danger has metastasised into a global threat that a loss of land won't mitigate.

"A military victory over one of modern history's most savage bands of marauders may yet prove pyrrhic."

A baton has changed hands. That IS dominates the news these days does not detract from al-Qaeda's contribution to jihad and the world of terror – "its great victory has been the spread of its ideology across a large geographic area", says former CIA chief Mike Morell.

That the US and much of the rest of the world continue to invest massively in endless security and counter-terrorism operations and that their graveyards are filled with thousands of victims of terrorist attacks and the post-9/11 wars confirms the success of Bin Laden's threat to bleed the West of blood and treasure. And a change in the discourse suggests an even greater tab – experts used to talk of it taking "a number of years" to see off terrorism, now they speak of operations that will be "decades-long" or "generational".

But the Foreign Affairs duo Mueller and Stewart daringly articulate a question, given that they were published just weeks ahead of the anniversary of the 9/11 attacks: given the American response over the intervening 15 years, does a terror group have to do much more than its initial act or acts of barbarism to earn long-term standing as a menacing threat?

Calling out the "serial alarmists" who repeatedly warn of al-Qaeda as an existential threat, despite the movement's failure in the US since 9/11, they warn: "This is all the more important given that history may now be repeating itself with [IS].

"Americans have decided the group constitutes a major problem. A poll conducted in the spring of 2016 asked the 83 per cent of its respondents who said they closely followed news stories about ISIS whether the group presented 'a serious threat to the existence or survival of the US'.

"A full 77 per cent agreed – more than two-thirds of them strongly."

In his book The Terror Years: From al-Qaeda to Islamic State, Lawrence Wright is forthright in setting out the cruel irony, or perhaps the wilful carelessness in Washington's decision-making in the wake of 9/11: "the 2003 invasion of Iraq by the US and coalition partners stands as one of the greatest blunders in American history – [IS] rose out of the chaos, throwing the region into turmoil that hasn't been equalled since the fall of the Ottoman Empire".

The White House proclaims that al-Qaeda is closer to strategic defeat and that it has IS on the run. These are statements of fact – but their measurement is an inexact science.

In the wake of the San Bernardino killings, US President Barack Obama told Americans: "The threat from terrorism is real, but we'll overcome it. We will destroy [IS] and any other organisation that tries to harm us. Our success won't depend on tough talk, or abandoning or values or giving in to fear. That's what groups like [IS] are hoping for. Instead, we'll prevail by being strong and smart, resilient and relentless – and by drawing on every aspect of American power."

If a lot has changed, a little has changed too. This is a contest in which fear wins until terror loses.