His closing log is clearly figurative, "this ship" meaning the Enterprise legacy, not the specific vessel he's sitting on. If he knows there's a successor sitting in dry-dock, this is even more apparent.
But the writers of
The Undiscovered Country had no idea that the completely different creative team that wrote
Generations three years later would make the decision to replace the E-A with a new ship. So at the time TUC was made, the line was not intended to be figurative. That's a rationalization of a later retcon.
Honestly, I don't know why GEN's writers made the choice to have the launch of the B happen in the same year as the retirement of the A. Since the movie was made 3 years later, why not set the prologue 3 years later?
Half-assed attempt at an in-universe explanation for Morrow's line: Maybe Starfleet was using a grading system based on the expected amount of wear and tear on a ship over time. He was saying that the Enterprise had put on 20 years' worth of wear and tear since the TMP refit. So it wasn't really the years, it was the mileage.
I don't understand this effort to contort logic to the breaking point in order to force Morrow's line to be accurate at all costs. It's so much simpler if he's just
wrong. Real people are wrong all the time. They make mistakes. They forget things. They have mental hiccups and say one thing when they mean another. Sometimes they just lie, to themselves or to others. So trying to force every last spoken word to be objectively correct is unrealistic. We should allow fictional characters to be as fallible and error-prone as real people are.
That seems to assume that they were new-builds, rather than refits.
Not assumption, deduction. There is canonical evidence for significantly more than 12
Constitution-class ships, as shown in the Memory Alpha articles I linked to before. There are more than two decades between "Tomorrow is Yesterday" and TUC, and zero reason to believe that Starfleet
didn't build more ships in such a significant span of time. In fact, given the heavy attrition of original-design Connies in TOS, I don't think there are
enough of them left to account for all the refit Connies seen in the movie era and after. So the most probable hypothesis is that they continued building new Connies. The assumption that they didn't seems arbitrary and unsubstantiated to me. Why wouldn't they?