> Existing miners can't do this in a PoW system

Sure they can. It's easy to only mine on top of blocks from certain sources, and if most of the network does this then those are the only blocks that matter.

>See my comment to another in the thread. Existing stakers can prevent new people from staking

There is little to no evidence of this. Completely unsupported conspiracy theory.

New people can still vote for their governments to affect those systems.

The distinction feels irrelevant

and UASF are a thing in PoS as well, if some entity/entities acquire a majority of the staked coins users can simply fork the malicious actors out. It's functionally equivalent except that it's an order of magnitude more expensive to acquire the required coins as opposed to getting the equivalent in hashpower.