What happens once every BTC has been mined?

So apparently the estimation is 2140 until all BTC is mined. So talking about the really long term, what happens once it’s all mined?

Who validates the transactions? ETH no longer can be mined, so that seems to have an eternal life.

But once all BTC is mined, no one will want to validate transactions for obvious reasons.

So does the original system of mining no longer be a thing for Crypto?

And what happens to all the coins that can be mined when they run out?

Has this happened yet to any coins, and what happened to the transaction due to no miner using their computer.

Issue is, since we don’t know who Satoshi is, he could be dead for all we know.

So if no one has control of BTC, it can’t be changed to POS, right?