I'd look at it like this:
Starfleet builds their hulls to last. The guts of a starship, on the other hand, aren't nowhere near as durable.
Consider:
Constellation: wrecked on the inside (3 month repair), hull scorched up badly, maybe a twisted warp pylon.
Enterprise: TWOK - TSFS: near-wreck, but on the hull all it had were scorch marks. This from some "unshielded" phaser fire.
All starships: a shot to, say, the secondary hull causes shorts and control panel explosions all over the bridge.
Therefore, The Excalibur, running with low to no shields for the "wargame", when fired upon by M5's full-power phasers did as much, if not more, damage than the Enterprise suffered when she was pegged by Kahn. Warp drive detonation (no warp core at this time), feedback takes out most, if not all, the control systems, and with it most of the rest of the crew, catastrophic system failure, including life support, costs the lives of the rest of the crew, all that's left is really a hollow shell filled with debris and radiation.
That would require pretty much a full rebuild of the Excalibur, as the only part still worthwhile is the hull.