Galaxy was a ship of exploration, and it got replaced by Ent-E due to the Borg threat..but Ent-E is still a good well rounded starship. Vengeance isn't--it is a flying weapons battery.
I see no reason to think the Galaxy
class was "replaced" by the Sovereign
class -- especially since we see numerous Galaxy
-class starships participating in key engagements throughout the Dominion War, yet we've only ever seen a single Sovereign
-class starship in the Enterprise