some beat up from Gene Roddenberry at the time about how the movie producers 'clearly' wanted to replace the Constitution with the Excelsior, but I've always chalked this up as being paranoia on Gene's part. Certainly, Harve Bennett has gone on record as saying it was never his intention, and the subtext in STIII is self-evidently that the Excelsior is supposed to be seen as the brash, bold new Starfleet ship whose overpowering arrogance is toppled by good ol' underdog Enterprise. All the bluster about "The Great Experiment" is just setting up the punchline of it failing to even clear Starbase, and despite Gene's fears it wouldn't surprise me if Bennett always
intended for 1701-A to turn up at the end of the next movie, even as he was writing the death of her predecessor.
The removal of 1701-D, on the other hand, always struck me as a cynical and unnecessary move, a decision made for purely commercial reasons. They wanted to capitalize on having a brand new ship in the next movie (merchandise etc), so 1701-D had to go. Personally, I'd have been perfectly happy to have simply seen it refitted with each new movie ala the TOS Enterprise, but there you go. The bottom line is that it's a dollars and cents business.