Depending on your sources, those classes have been called all kinds of things (explorers/cruisers/battleships/etc).
To my mind, I don't see the Sovereign as a follow-on successor to the Galaxy. Two different types of ships with different mission requirements and specifications and different roles.
The Sovereign class isn't a "new and improved" Galaxy class. It's a different kind of vessel with a different role, better in some ways, not in others. Sovereign doesn't necessarily make the Galaxy obsolete. They might even serve well together in a complimentary manner.
I agree with this.
The Galaxy Class to me, represents an ambition to be "the best of everything" It was huge, luxurious, well armed, well equipped, and was in the late 2370's the full realization of Starfleets potential in ship-building.
The Sovereign on the other hand, was built only 10 years later, but in a vastly different time for Starfleet, recovering from a costly war, on edge, just a different time.
I feel that the Sovereign is a product of it's time, it seems more suited for battle, is stripped down, does not seem to have families or "superfluous" features.
Products of different era's in my opinion, but the Galaxy is still a very new ship really, I don't think they would make any more of them in a post war era, but the ones that already exist are still top of the line ships, and still would have a very important role in long range exploration.
Galaxy = "ultimate potential"
Sovereign = "realistic necessity"
that's how i'd describe them, both beautiful too.