I had assumed that either Excelsior would be renamed Enterprise or that a second Excelsior class ship would be revealed as the new Enterprise (which is what they really should have done, IMHO) but I had no idea when this would happen.
I agree. The Excelsior
was supposed to be a state-of-the-art design. The Constitution
-class design was being phased out of Starfleet by 2286. Why offer Kirk command of an obsolete ship?
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.
"Canon is only important to certain people because they have to cling to their knowledge of the minutiae. Open your mind! Be a Star Trek fan and open your mind and say, 'Where does Star Trek want to take me now'." - Leonard Nimoy