Proof-of-stake (PoS) protocols are a class of consensus mechanisms for blockchains that work by selecting validators in proportion to their quantity of holdings.

This simply means that the richest, usually the first comers, can continue to control the network at no additional cost, other than opportunity costs.

Proof of work doesn't allow that. Of course, the richest could continue to control the network, but only by continuous investment in the network.

Anyone can join the Bitcoin network, without even purchasing BTC, just mining. This is not true of PoS. PoS almost creates a permanent oligarchy