Interesting article. How much support does ISIS need among the rank and file of Islam? If 1% of Sunnis support them and provide money, arms and help recruit manpower, they can stay in business if not become stronger. I think the key is that the stated goals of ISIS need to be frustrated and that fact trumpeted to the masses. Lack of success is a big buzzkill.
But there is some evidence they receive tacit support from the Saudis as well as other countries like Tunisia. They can still recruit internationally. 1% of 1.5 billion Muslims 15 million people. And the disaffected among Islam in Europe? You only need a few, and one with access to weapons to create chaos. It is impossible in a national society like France to sort through all the potential perpetrators. Any success can be trumpeted. Any success is a win to them. And when you believe that dying for Islam gets you to heaven, it is damn hard to stop.
I don't think ISIS is going away any time soon. there will be bloodshed and terrorism into the foreseeable future.