If Obama never tried to overthrow Assad, this problem would not exist. Obama had no real plan to overthrow Assad, no plan or guarantees for what a new Syria would look like. This was Obama's war, with a lot of help from the late Sen. McCain.
Defeating ISIS, leaving Assad intact and crippled, is the best the U.S. could get out of this. Trump got it and is getting out.
Defeating ISIS,
If by “defeated” you mean 20,000-30,000 disbursed fighters then yes ISIS is no more and might as well include a ‘defeated’ Al Qaeda who has another 50,000 or so disbursed fighters in Syria.
Of course, once the US is gone, they’d never think of reconstituting and begin planning for a major attack on mainland US? Of course not and don’t be silly, they are defeated and will go home hanging their collective heads in shame, right?