When we toppled Hitler we went in and occupied West Germany and embarked on a successful nation building program. 
We did the same thing in South Korea.
We tried it in Iraq and it's a disaster.
We didn't try it in Libya and it's a disaster.
If we topple Assad the civil war will still not be over, as the various religious sects and ethnic groups, and outside actors (Iran, ISIS) vie for power.
Topplel Assad and you may end up with something worse, unless you're willing to occupy the country and embark on another nation building project.
Are you?