The more you keep people bottled up with the headchoppers, the more likely they're going to either join up (to keep from getting chopped, because a russian airstrike gib is later and today is now) or turn into their own bunch of 'em. Either tends to end up a net negative for everyone.
I suppose the first part of that makes sense, and I admit it's something I didn't consider, but the difference in being conscripted into a militia and joining/forming one willingly is pretty big to just gloss over. I'm still not convinced that making entry into the USA, where very few of them are even trying to go anyway (compare to those headed for Turkey, other Arab countries, safer parts of their own nations, or Western Europe,) more difficult is going to push people who fled a conflict to just go ahead and join the people who forced them out in the first place instead.
The real problem is that there's no practical end-state for that. Either you're accepting (eventually) the entire population of every country so afflicted along with plenty of infiltrators, or... what? You try to do so but don't achieve perfect efficiency? That's why I'm partial to attempting to solve the problem in a semi-competent way-hearts&minds is the right mindset, and it did a lot in Iraq and Afghanistan in the relatively few instances where it was done properly (this is why you need training for troops doing this shit, because poorly done military intervention is worse than nothing).
Could also play the hindsight game and suggest that supporting violent extremists and helping religious zealots overthrow governments ain't a great idea, but that's in the past.
Also worth noting that the refugees heading to the U.S. and Western Europe aren't generally the most desperate, but rather the ones who already had sufficient resources to fund that sort of exodus. A whole damn lot of refugees were taken in by countries much closer to their homes. Never mind that the majority of refugees from the "Syrian civil war" aren't Syrians or from contested areas.
This Trump fuckup just goes to show that there's bullshit propaganda on both extremes that needs to be filtered out if we're to have a reasonable discourse. The execution was shockingly incompetent and heartless, but the list of countries is pretty much identical to Obama's 2015 visa restrictions.