But ISIS won’t let the civilians leave. There will be a huge humanitarian crisis which will make the situation untenable.

Iraq isn’t a USA protectorate any more. There are powerful shia groups that resist USA involvement and the prime minister has his hands tied in many occasions. It’s easier for Iran to get involved than the USA.

I generally agree with your perspective but I want to comment on a specific point. Taking control of Aleppo would be time-consuming and costly in lives. The SAA has demonstrated some weakness when it comes to close combat, house by house fighting against jihadists.

Boomerang. Because it hits and then always comes back.