So this made me think of spaced repetition, which is based on how the brain retains memories. I wonder if you could make a cache retention algorithm similar to what the brain uses.

I'm thinking something like this:

Every entry has two values "solidity" and "time since last seen". When an entry is accessed solidity increases based on time since last seen - if it was a short while ago it increases just a little, longer means bigger increase. This is to ensure that solidity will correspond roughly to the long term need for the entry and doesn't go up too much if something is accessed a lot of times in quick succession.

Entries with low solidity and long time-since-last-seen are evicted.