My point is that every word of this is speculation (along with the idea that they were mothballing *all* constitution class ships in another post above, definitely not something said in the films), and builds inference on inference just to make it so that that one time someone said it was originally the Yorktown (apparently Gene? Who was basically shut out of the process of making the films at that point anyways) make sense. It's a conclusion-driven theory, begging the question.
Anyways, I'm gonna leave it at that. It'll just go in circles from here.
I'd agree that it's speculation, but it's the theory that has the most weight to it.
When the Enterprise-A was revealed at the end of Star Trek IV, it wasn't clear where it came from. Did Starfleet build an exact replica of the Enterprise just for Jim Kirk? Or did they slap a new coat of paint on an old ship? It's never said, exactly.
But when Kirk was young, these ships were the top-of-the-line. Young Kirk doesn't give a damn about "old people" starships, he likes this new one because it's the latest and greatest. By the time of Star Trek III, Kirk is an old man (although he's feeling much younger), and then Starfleet tells him his ship is old. Starfleet is showing off it's new hotness, the Excelsior. People like Kirk and Scotty used to be edgy and cool, but now they look at their once-edgy old ships as "classics", and don't like the look of the new ones. The Excelsior is treated like an enemy. It's classic "old people" syndrome.
By Star Trek IV, they've cooled down a bit on their opposition to the Excelsior (it's not being
actively painted as the enemy), and Sulu actually wants one (Scotty still calls it a bucket of bolts). It makes sense why Kirk wants a Constitution, but it doesn't make sense why Starfleet would build one, not while they still have several of them in service, and plenty of young people ready and eager to climb onboard the newer ships (even Sulu).
And then, by Star Trek VI, Kirk's Enterprise gets mothballed... again (Spock: Go to hell). And in very short order, "Enterprise" is reborn again as an Excelsior (Generations).
It seems as if most people are on the same page about the Enterprise-A, that it was a minor stopgap ship, probably secondhand with some upgrades to keep it relevant, declared to be "the Enterprise" in Jim Kirk's honor. It was "obvious" to some from the very beginning (it's the lowest-cost, fastest turnaround solution to finding a present for Kirk), Gene seems to have thought that's how it went, it seems to fit with the themes of some of the movies, and several different movies (with several different writers) seem to line up and make it seem logical. The Enterprise-A has a different interior, but that would seem to miss the point of a replica (Kirk: I miss my old chair...), and fit with the idea of finding a used car that's a close match for the old one.
I don't think Starfleet specifically ordered the mothballing of all of their Constitution class ships all at once, but after the Excelsior was introduced we saw two big examples of Starfleet refusing to repair spaceworthy Constitutions, choosing instead to mothball them. I think it's obvious that they intended to phase them out in favor of Excelsiors.
From a pure "plot down, watch an episode" standpoint I think Voyager is more watchable, just because while Janeway might act like a crazy evil person sometimes, Archer was just consistently annoying and boorish.