The Sovereign-class appeared to follow very quickly after the Galaxy, given what we know about about how often Starfleet seems to upgrade its ship classes (rarely). I've often thought that perhaps the Sovereign-class was out of sequence, hurried into service because the Federation needed a hard-hitting capital ship to compete with the Borg and Dominion.
It might simply be that the Sovereign
is not intended to replace or succeed the Galaxy
, but instead is intended to replace/succeed some older vessel - say, the Excelsior
, which it greatly resembles.
Just because Picard moved from a Galaxy
to a Sovereign
doesn't mean squat about which class is supposed to follow which. Picard might just as well have been assigned to an Intrepid
or a Miranda
, and that wouldn't have made those "successors" of the Galaxy
class, or even successors of the E-D.
Sisko moved from Danube