From my very limited understanding, with Bitcoin the separation of powers comes from the miners ability to reject an update. With ETH Proof of stake, that is less clear - can the people staking refuse to accept a new protocol version/update?
The people who run the validators can, but validators on the old version will be incompatible with the rest of the network (if the rest of the network did update), and they will not be able to sync. It's no different from Bitcoin in that sense.
