Admiral Morrow explains to the Enterprise crew that the ship is going to be decommissioned, because "her day is over" or something like that. He doesn't specifically mention a museum, but it's clear that the ship is considered too old and outdated to be in active service.
If you recall, the Excelsior class was already in service by this point, and I think it was intended to replace the Constitution class (more or less).
The Connie was an older ship design which was refit to include modern technology, but the Excelsior was designed and built much later... probably improving on things that Connie lacked.
Considering we saw the Excelsior class in use for over 100 years, no ship can get 'too old' if its continuously upgraded and refreshed over time with modern technology and standards.
I think back then, Admiral Morrow may have said that because plans for upgrading older ships for longer periods of time may not have been drawn out.
Replicators didn't come into the scene until sometime in the 24th century.. and with the Excelsior class being still in service, it may have 'crossed the threshold' where initial replicator tech became available, and it became easier to upgrade older ship designs to latest standards.
And who knows, its also possible the Constitution class received similar design upgrades in the 24th century and was then brought out of 'hybernation' (so to speak).
Or, Starfleet may not have revived the design until sometime in the 25th century, giving it a complete overhaul to bring it again in line with latest standards and morwe frequently upgraded from that point.
The Enterprise was designated as a vessel for trainees, this meant to me it was close to becoming decommissioned. I hated the notion the Enterprise was old since TMP made an effort to give hints the best was yet to come...
Correct me if I'm wrong, but I don't recall anything from canon (TV shows or movies) saying the Enterprise was designated as a vessel for trainees.
As far as I recall, this notion was entertained by unconfirmed sources, but officially (from canon point of view), I don't recall anything being said that confirmed this.
Hey, Indiana Jones said it best, "Its the not age... its the mileage." Besides as this thread presented, what was the point in refitting the Enterprise? It was a lot slower, it barely worked, engineering team had to wear radioactive suits, and it gets destroyed in the 3rd movie. Best to make the movie version its own reality and leave TOS out of this. Its not the same Enterprise, the movie's Constitution Class / 20 years old vessel "talk the talk" but couldn't "walk the walk".
Any vessel receiving frequent upgrades while in active service will eventually overcome intial inefficiencies.
And, wearing radioactive suits in the 23rd century may have been a necessity on ANY starship of that era until a better warp core material shielding was developed (which would be integrated on other ships - and combined with internal forcefield technology perhaps). Perhaps they used different methods in M/AM power generation which required use of radiation suits in close proximity to the core in that era.