Obama made the calculation that replacing Assad with Al Qaeda was a good trade, mostly because the Saudis and Turks assured him they could be controlled or replaced later. Unfortunately for everyone, they started slaughtering indigenous Christians which made it impossible for normal Americans to accept this policy. Turks didn’t care, but we do.
Plus, it didn’t make much sense to overthrow Assad, Iran’s number one ally, while helping Iran get a nuke.
If you want to overthrow Assad as a strategy toward containing Iran, it starts to make some sense. But again, not if your replacements wiped out every minority tribe in the entire region and hate you just as much as Assad ever did.
Christians end up with the short end of the stick in scads of global conflicts.