I think the best option would be tell a user who is about to resurrect a conversation that has been evicted from cache that the session is not cached anymore and the user will have to face a full cost of replaying a session, not only the incremental question and answer.
(In understand under the hood that llms are n^2 by default but it's very counter intuitive - and given how popular cc is becoming outside of nerd circles, probably smaller and smaller fraction of users is aware of it)
I would like to decide on it case by case. Sometimes the session has some really deep insight I want to preserve, sometimes it's discardable.