I guess it all depends how much you want to admit is a reflection of real-world developments of the series, and how much can be retconned away.
Which I think is a primary tension between many Universe-based fictional works. People draw the lines at all points.
Onto the fun:
Miranda - designed to be simpler to build, shorter range, and able to do single missions at a time. Possibly a ship you send because you don't want to waste a Constitution, but you need some room for mission payload. Because of it's weapons you'd want it patrolling hot spots or in a fleet engagement. But it doesn't have the damage control capabilities or endurance of a Constitution.
Constitutions, though a Starship like the Mirandas, would get the BEST sensors, computer, data storage, communications and linguistic facilities, and even some of the best food prep and storage... and best rec rooms.
Why are there Miranda's years later? Well, probably because it's still able to do that shorter range, limited scope mission profile. And it's still cheap to build. They could be easier to maintain.
On the other hand, the Constitution isn't able to perform the same roles it did - requirements for galaxy exploration and ship of the line keep increasing! And it's too expensive to have around doing limited missions, because there's all that equipment you don't need anymore (science labs, sensors, linguistics).
I suppose they probably did have a bunch of Constitutions slowly die off - but if I were Starfleet, I would have deactivated my Constitutions to make good on the Klingon peace treaty so I could keep building Excellsior class ships =).
Why did the Reliant do so well?
I think
- 1.) First shot
- 2.) As a training ship, the Enterprise would have novice crew, but I think they'd also have LESS crew than a Constitution normally would have. So you have fewer experienced engineers (and there's a few older ones in the bunch, it's not just Scotty!) than normal, and you have a bunch of cadets, so that really puts a damper on your damage control.
- 3.) As a training ship, the Enterprise may have been pretty brittle at this point. It's put out to pasture. Maybe the phasers don't put out as much juice as they used to, or maybe the hull plates and the shields aren't as effective as they once were. Perhaps they're externally similiar to Reliant but Reliant's systems are 18% better. You don't nessesarily upgrade your training ship's phaser banks or shield generators to the new design specs when it comes in after a cruise. You send those resources to new ships on the front line when THEY come in. **of course, I can also see that Starfleet may have a rotating training ship thing**
^^I think there's a lot stacked against this particular Constitution class ship, in this situation.