That is the $64,000 question. Personally, I think ISIS will collapse from within, and even if there may be 30,000 foreign fighters I don't think more than a quarter of that will be from Western Europe. Burning the pilot was a very bad move.
Too much is being made of this. They'll probably end up making tea. In many ways if ISIS are really fighting the good fight and demanding jihad then to have only got a few thousand of the many millions of Western Muslims is a bit of a poor turnout really.