Captain Picard's Hair wrote:
Do we really know that the Connie is that much more powerful than the Miranda, though? They seemed pretty evenly matched in TWOK.
It's fairly more, not leaps bounds, but a bit, at least if we trust the numbers on the DITL site any. The connie is faster, and has a slightly higher combat score. And the battle in TWOK really isn't too much to go on, because it was an unusual fight. Both ship's first tricked the other so they could score unshielded hits, then they went and hid in a nebula. That fight was much much more decided by the tactics involved than the ships.
The Miranda is a more compact design, though... that may have made it easier and cheaper to build and maintain.
Numbers certainly could have lead to there still being mirandas around, and again, that compact design should have been more sturdy, thus the hulls lasting longer.