A big problem with that theory is that it was established in dialogue that the Miranda class (i.e. the Reliant) is faster and more heavily armed than the Constitution class (i.e. the Enterprise).
That doesn't sound right. In "The Wrath of Khan", it was established the lightly-damaged Reliant
was able "to out-run and out-gun" the heavily-damaged Enterprise
We do not know how the two ships compare to each other in their undamaged states.
We do have some hints though.
1. The Enterprise holds the speed records that the Excelsior aims to break. She likely has more power and thus more power for phasers.
2. The Miranda-variant-with-rollbar, Reliant, has the same number of phaser emitters as the Enterprise and possibly 2 extra aft-torpedo launchers.
3. The Mirandas that we see in TNG are mostly the ones without the rollbar which reduces the ship's phaser emitter count by at least 4 and torpedo launcher count by 3 (moving the launcher to the saucer underside.)
4. The Dominion War saw the Miranda-variant-with-rollbar return in number.
A key point in "The Undiscovered Country" was the mothballing and retirement of the military program of Starfleet. The Enterprise and her type could've been part of that group. The Mirandas by design could be made into a non-threatening explorer by removal of the rollbar. So it would make sense for Starfleet to stay flexible with keeping the Mirandas around but retire the more dedicated warships like the Enterprise, IMHO.
Wait, when was the Enterprise ever a dedicated warship? She was an explorer and patrol ship with 14 science labs aboard, and numerous first-contact missions to her credit.
Seems the Mirandas with the 4-tube torpedo pod and heavy rollbar phasers is the more warlike ship.