Added to this is the wealth that ISIS amassed during its caliphate phase. In press reporting in 2015, a U.S. Treasury official estimated that ISIS had about $500 million. This is well beyond anything possessed by al-Qaida, which was always scrounging for money. I doubt that experts today have any confident estimates of the degree to which this wealth has shrunk, and it may still give ISIS leaders the flexibility to hire experts, buy weapons and deploy operatives. On the latter score, ISIS has easier access to the West than al-Qaida did. The 4,500 or so Western recruits drawn to ISIS — we must assume that a sizable proportion of them survive — give it the ability to move among us in greater numbers than al-Qaida ever managed to.
Second, the original causes for the growth of ISIS remain unaddressed and will likely keep the extremist impulse alive in Syria and Iraq. ISIS thrived because the grievances of Sunni Arabs who rose up during the 2011 Arab Spring demonstrations were violently suppressed in Syria by the Assad government, monopolized by the Alawis, a non-Sunni minority sect. This drew Sunnis from throughout the Arab world to Syria at a rate of about 1,000 per month from 2014 to 2015, many of whom joined ISIS or al-Qaida affiliate groups. Now that Assad, with the help of Russia and Iran (a Shiite state), is once again in command of much of the country, these grievances will continue to fester and likely draw adherents from the region to any opposition movements that survive or revive. And in Iraq, broad discontent with the government, manifest in growing public protests, will also be fertile soil.