Hacked mining pools are a non issue. As soon as the hack is detected the miners will leave and a new mining pool will be made. In fact, mining pool has been hacked in the past many times, and a new one always takes its place.

“You might now say “well, for PoS you can just hack the private key and this is less hard than to hack a server running a mining pool”. The truth is, that just depends on key length.”

No, you just have to hack into the computer that contains the private key, this has nothing to do with key length and cryptographic security.