CT Would the emperor being destroyed have been necessary in the long run given his age?

Sidious had been a Sith for many years before he became Emperor; the Sith have been around for thousands of years in the shadows prior to Sidious. Sidious needed the Imperial machinery to hold power - hence his and the Sith working toward that aim. Without that he is going to have to go into the shadows again. If the machinery of the Empire is destroyed then Sidious can no longer rule.

I guess another way of asking the question is, does evil have to be destroyed in order to defeat the Empire?

 

Yes. The Alliance doesn't have the means to topple the Empire completely as long as Palpatine is alive. Within one year the Empire falls apart and signs a treaty ending the war. In Legends it was decimated for four years, then was whittled down to a small territory sixteen years later.

The Saga was long predicated on the idea that good (the Jedi and the Alliance) had to destroy evil (the Sith and the Empire), in order to free the galaxy.