I don't see how switching consensus from a proof of work system to proof of stake system "is entirely within the scope of a PoW system". If the majority of the network decides to do that, I understand why that makes it valid, and this is intentionally hyperbolic/don't think proof of stake is properly viewed like this, but to me that reads like "voting in a dictatorship is entirely within the scope of a democratic system". Technically it might be true, but the moment you make that valid democratic decision a decision like that can't happen the same way again.