We don't have to consider the Sovereign
class the successor of the Galaxy
class in any sense. For all we know, Galaxy
is still going strong - and it just so happens that the lifespan of starship classes has been gradually increasing, so that the E-D could theoretically have been succeeded by an E-E of the same class. And the E-C could have been succeeded by another Ambassador
(a type with a lifespan of perhaps sixty years), but Starfleet didn't give her a successor until after the days of Ambassador
were over. But the E-B was a shorter-lived design (say, forty years *), and was already old news when the time came to name the E-C.
*) I mean, looking at the registry numbers and the ships carrying them, one can approximate a progression of a thousand NCC numbers per year, or at least ten thousand per ten years, in the 24th century. We witness ships of E-B ilk having a coarse range from NCC 10k to NCC 40k, or forty years during which these things are being built.