It's not an exaggeration to say that they're fleeing from genocide. That the only thing stopping a small-scale genocide from going large-scale is the sheer number of people who have fled. Or that it's unreported because Islamic State kills reporters.
My country unfortunately turned back a ship load of Jews fleeing the Nazis, when they arrived on our shore in WWII. Most of them soon died in the death camps. It would not have made us look better to have said, "But you'll be needed to eventually rebuild Europe!"
Let's not also forget that pre-ISIS conditions were awful and dangerous. Even if the Islamic State were to magically disappear, things would be even worse than before they formed. A big cluster of civil wars between the Sunnis, Shiites, Alawites, Kurds and Christians. There will be open war between a now more or less independent Kurdish area and Turkey, which has been bombing them rather than ISIS. Bashar al-Assad, now backed by Putin, will still be targeting the rest. Iraq will still be hell, even worse than before.
Any "eventual rebuilding" is a couple generations off, not that anyone will fund it. And by then there will be
other major problems destabilizing the region.