I think the case of the “Beatles” disproves your theory, I know you posted “most” but that’s a very subjective opening polemic.Most ISIS fighters from here come back disillusioned rather than dangerous. The foreign fighters are generally not trusted by the hard core Arab fighters and generally end up cleaning the bogs, brewing up or as in Begums case getting used as a sperm receptacle. Most would be back within days, but the first thing IS do is take their passports off them. They sell a myth like a 30's Errol Flynn movie Bollywood style on line and the reality for the foreign wannabees is obviously disappointing
A lot would have come back disillusioned but that’s because they eventually got a right twatting, which was fully deserved.