Bear with me here, but I strongly suspect that this is not the case. Because PoS doesn't need miners, it seems the system can transition to PoS at any time and there's nothing miners can do to stop it. So unless the miners also happen to control over 50% of the staked ETH supply, miners don't have a say at all.
EDIT: Don't get me wrong -- I wish Ethereum could only upgrade with overwhelming miner consent. It would check the power of developers to alter the monetary policy as capriciously as they have done so far.
This is actually a current issue that's being discussed because the transition fully relies on the miners to make hand off the transition because of the contention with moving to PoS. One possibility is that miners will fork the chain, although that's not necessarily a huge concern because the community will follow the PoS chain. Another possibility, and I'm a bit fuzzy on the details, but it has to do with them re-mining the same block over and over so the transition block never happens (or something like that).
Bear with me here, but I strongly suspect that this is not the case. Because PoS doesn't need miners, it seems the system can transition to PoS at any time and there's nothing miners can do to stop it. So unless the miners also happen to control over 50% of the staked ETH supply, miners don't have a say at all.
EDIT: Don't get me wrong -- I wish Ethereum could only upgrade with overwhelming miner consent. It would check the power of developers to alter the monetary policy as capriciously as they have done so far.