> the stakers only need to make an investment once in the beginning

> The whole system could collapse overnight like a domino if something goes wrong

IMO, these two things are one of the main points of tension which PoS uses to operate. The stakers have put up a significant amount of value, and it's in their interest to maintain that value by preventing things from going wrong.

Not only that, but they are actively pitted against each other -- it's in their interest to find any node that tries to violate the rules, and submit an attestation slashing the offender's stake for bad behavior.

There are also a number of incentives (such as the upcoming EIP-1559 upgrade) which are designed to align incentives so that cartel members are encouraged to break away for their own advantage.