Nothing could stop the exeter from being refitted to TMP form, STO tech is magical.
Perhaps you should start by not using the "Magic" excuse because that is not a argument, its a word that is used to excuse anything and everything.
The Exeter was abandoned, its possible that Starfleet send a team to recover the ship and it continued service but that is a conjecture, since the ship was abandoned in orbit its also quit possible the ship orbit decayed and it crashed on the planet or considering the hazard posed that the Enterprise being unable to tow the ship or manned it had activated the ship self-destruct to prevent the above scenario as it posed a serious hazard.
All are conjectures, however we do know the Constitution refit was extremely problematic and the Exeter being refitted would simply amount to being another ship that would serve for some more years (like the Enterprise) until it was going to the scrapyard.
Also TMP is certainly NOT T5, even the Miranda that was a newer ship does not have a T5 variant and never will, also using the excuse that "well its new so it can perform to T5" is the same as saying any Coast Guard ship that is recently can perform as well as a Nimitz Class carrier since its "newer", the fact the Exeter can use the Constitution Refit as a costume makes sure that ship will never be a T5 ship in the game.
Adding a few hull points a few shield points
1 console slot isn't enough for a lot of the fleet
Ships we have already
The galaxy class is a shining example of this
It still a poor ship choice,Brandon your crew must
Do better than this of you want these ships to sell
Just look at how poor the galaxies boff arrangement is
Set this ship up I'n a tactical mode and it will sell
A excelsior fleet ship would be nice
Make it's LT Cmdr Tac into a commander, it will sell
The tier 1 ,2 ,3 ships as a fleet tier 5 would be nice to
Use for helping low level players get thru missions and
Mirror events, I think they should have the 12 boff/ 4 or 5
Add 1 more weapon slot 1 console slot some hull
And shields and it would be a fun ship to return to
The story missions with
All of the empires would have lots of these ships I'n storage/mothballs
And to rework refit a few would be common
A idea to work with here, captured enemy ships reworked
And used as a tier 5 ship including lockbox ships
However these ships would be inferior to the original
I'n closing make the current fleet ships better and they
Will sell especiall the canon ships.
I'm also in favor of a Fleet Exeter with a TMP costume option. As that doesn't have any relation to the 'no T5 TOS Constitution in STO' dictate from wherever that really came from.
If it looks exactly the same, then it is a T5 connie. So unless CBS changes their mind, there will never be a T5 ship with the exact same costume as the TMP connie(or TOS connie). However the Exeter and Excalibur costumes are completely different.