OK, thanks for the elaboration.
How would a POW network adjust if someone were to acquire a majority hash rate? Even if they migrate to a new chain, how could they prevent the hash rate owner to use and have the same influence on the new chain?
In regards to POS, the things you pointed out actually did happen last year - the exchanges staked their STEEM and helped a single actor take control over the network. So all of us forked over and created the Hive blockchain. And we implemented some tweaks - when someone stakes their tokens (which allows participation in governance), it takes one month before their stake can be used for governance decisions. So this should prevent exchanges from interfering with governance.
And again, even if someone manages to take control of a majority or very substantial stake and acts in a way that the community doesn't like, the community can create a fork and make it so that the new chain doesn't have that actor's stake.
Hive has done a great job at showing the way forward for POS chains.