[armchairhacker]

Is there a mod that gradually adds new resources to old worlds, gradually adds ores back to their gaps in caves, gradually restores the environment, as well as gradually removing torches and “decaying” old structures?

Otherwise someone should make one. No need to full reset servers any more.

[HomeDeLaPot]

I think some servers do "chunk resets". The idea is to take chunks with little or no player activity and delete them so they'll be regenerated next time someone comes along. I think I've heard of this being done automatically (chunks that a player hasn't been to in X days will be deleted). It would lead to a discontinuous world though, with older chunks having sharp borders with newer chunks that aren't even the same biome.