Agree that cleanup drones aren't adding much. Mines are pretty uncommon and I'm not sure how the AI ever repairs them from remains.
The salvage + reprisal mechanics are intended to solve two problems at once. The first is that when you lose a fleet, it can take a very long time to rebuild all those ships, because your metal cap doesn't really keep pace with the total cost of your fleet.
The second is that losing a fleet didn't feel particularly dangerous. The usual response was to put time to +10 and wait until your fleet was back, then throw it back into the grinder.
The reprisal + salvage mechanic ensures that when you lose a fleet, you're now in danger because of it, but if you survive, your economy gets a boost that allows you to refleet faster.
Admittedly, doesn't feel great that the AI kicks you when you're down, but I tend to reload after stupid fleet losses.
If you could disable reprisals, that option would probably also have to disable salvage.
The salvage+reprisal mechanics do have some rough edges, certainly. I'm working on yet another attempt at a Neinzul themed expansion, so I have full caps of Enclave + NCC starships. I see reprisal waves *just* from the drone spam. Unfortunately, the Neinzul Youngster only sends younglings as reprisal waves, which means they don't actually give me any salvage, they just suck. The other AI just happened to unlock Youngling Fireflies, too, and it's very fond of Firefly waves, so I've gotten hardly any salvage this game, even using my homeworld as a whipping boy all game.