Honestly, the thing kinda breaks if you have several sets of rules aka Bitcoin clones. No, ideally, the thing works if you have > 50% computation resources of the planet consumed (and of course, controlled) by miners using one set of rules. This way you cannot have competing chains at all, and the correctness of the set of rules is kinda obvious - they're correct if they make sense to me (paying arbitrary rewards to oneself does not make sense, obviously) - no need to ask anyone - I see an enourmous chain with enourmous burned computation and I see the code.

In this sense there could only be Bitcoin. PoS cannot come close to that at all.

P.S. To clarify a bit more - I don't think the most-difficult chain can be with broken nonsensical rules, not in the long term.

Choosing between Bitcoin and Bitcoin Cash is a completely different matter. You choose whichever you like, and you can verify yourself that the software follows those rules. There's no chance that users wouldn't converge into their chosen blockchain, because both forks follow the longest chain in their forks.

You assume that the community somehow can agree which chain is the correct one in proof-of-stake. This is not true because there's no longest chain rule. Without an authority, users would never be able to converge into the same chain, after a split has happened.