Defeating the villain is rarely enough. Building is harder than destroying, and it is very unlikely that something good will spontaneously fill the void when something evil is taken away. It is also insufficient to speak in vague generalities about the ideals to which the post-[whatever] society will adhere. How are you going to avoid the problems caused by whatever you are eliminating, and how are you going to successfully transition from evil to good?

In fantasy novels, this is rarely an issue.

Two exceptions: Orwell's Animal Farm, which was explicitly an allegory for the Russian Revolution, and Brian Sanderson's Mistborn trilogy, in which defeating the Evil Overlord turns out to have been only the beginning of the heroes' troubles...