What evidence is there that the Constitution-Class was being phased out at the time of TVH? Yes, the Excelsior was state of the art, but it was also an experimental prototype that was just getting ready for "trial runs" three months prior in TSFS, when it was sabotaged by Scotty. It would likely be years before the Excelsior class was put into full production and the Connies phased out of active service. So, I don't see it odd at all that new ships of that class were still being produced at this time.
Admiral Marrow's comments gave me that impression. Why decommission Enterprise
-class vessels are to continue being used? She was badly damaged but salvageable. Why not do another refit as Scotty suggested? Also consider that no other Constitution
-class vessels are seen after the Enterprise-A
is retired from service in spite of there being several Miranda
-class ships that remain active well into the twenty fourth century. The only on-camera explanation that makes sense is that the remaining ships were retired. It may not have happened at the exact time I'm suggesting, but it's likely production of new ships stopped in the late twenty third century.