Does the Empire really have space superiority?

Does the Empire have enough space superiority to force the Rebels onto vulnerable planetary bases?

No; Imperial ships are too thinly spread across the galaxy to prevent a mobile carrier force from using remote or uninhabited systems.

The Empire does not enjoy meaningful space superiority across its entire territory. Even a large number of Imperial Star Destroyers is small compared with the number of systems and routes in a galaxy-wide state. A Venator does not need an inhabited planet because it can use remote systems and recover resources while remaining mobile. The number of ships therefore has to be judged against the territory and the number of systems they must cover.