If Bashshar were to step down

Of course, I have been in favor of the Asad regime stepping down before it even came to power in 1970 but was thinking analytically about the conventional wisdom: why is the conventional wisdom out there is that once Bashshar steps down--if he steps down--all will be well, and all the fighters of ISIS and Nusrah would shave their beards and join pro-Western democratic and liberal and secular parties in the new Syria. Is this not the current assumption in ally Western writings about Syrian scenarios?