Now: What if e.g. 99% of miners blacklist an address or UTXO, would the current system be enough? Would there be a way to improve that, introducing new protocol rules?
What if it's only 51% of the miners, and they also ignore any blocks that include "forbidden" transactions?
3) Any node ("challenger") can challenge a recent block proving that the miner hasn't followed rule 1 or 2, and if he can prove it, then the "challenger" node gets the block reward and fees (this would need major protocol changes, but should be possible as similar mechanisms exist in "slashing" PoS protocols).
That's easy to get around if the challenger colludes with the accused.
The current mining system has worked just fine since it was created. At the moment, I see no reason to even suggest to make changes.