I don't know that Syria is actually a big problem for me, or for US National Security. I mean sure, you can rightfully argue that ISIS will breed new terrorists, but I'm not convinced that they are any better or worse than Al-Qaeda. One of the major reasons that they were able to conquest over so much land in the first place thruogh Iraq was because there was a huge amount of American military equipment that the Iraqi 'Army' left behind after we had given it to them to fight Al-Qaeda.
I'm not worried about the Madhi prophecy because most people just see it as a weird religious sect, and not a real prophecy. Nor do I believe that most ISIS fighters actually believe in that cause or any cause in particular other than, "We've been getting fucked over by everyone from the West to the Russians to our own 'Leaders' and we are sick of it." Many fighters simply believe it is the best paying job in the area. Happens with the Taliban all the time where guys from the ANP just switch over depending on who is in the territory at the time.
But the main reason I'm not worried about ISIS is that they don't pose a real threat to me. Sure they can kill people with a truck in France and England, and that could definitely happen here. San Bernadino happened here even and there was also this other thing that happen in September but I can never remember what day. And even after all of that, the losses and money spent on fighting that, ineffectively and counter-productively, has been far greater than any loss of life and property lost to terrorism.
So I don't know what will happen if Assad falls (and I don't think he will at this point), but I also don't want the responsibility of caring for it. Europe isn't doing it, China isn't doing it, South America and Africa don't even play on the world stage. Israel isn't doing it. Why is the US the only country that sees this danger as an existential threat? I feel like it's more of a child throwing a temper tantrum and we just let them get tired and fall asleep.