Question about Proof of Stake

Have been observing and investing on cryptocurrencies from 2020. but only recently have started studying it . Quick question on Proof of Stake.

Lets say a super rich entity ( mega billionaire or government, goes ahead and buys 51% of all Ethereum available)

then this person now has 51/100 chance of being chosen to propose a new block.
whats stopping this entity from tampering with the chain, for example lets say start restricting transactions for some reason. not picking selective transactions from the mempool. and essentially restricting them. how is this prevented???

also since this entity gets 51% of the transaction fees, its only going to accumulate and get stronger through time and eventually end up owning the entire network.

im sure the eth community would have thought of such a scenario before implementing the PoS .
Can someone help me understand what are the counter measures for such a thing ?

Thanks