The difficulty bomb makes it so that consensus doesn't have to occur to switch, but has to occur to prevent the shift from PoW.
Everyone doesn't have to agree. There will be miners that attempt a fork to keep their position. There is nothing wrong with this, it's how this stuff is supposed to work. But the fact that PoS has been on the roadmap since the beginning takes away legitimacy from any claim that the PoS transition is illegitimate. The fact that Ethereum Classic exists and is a PoW version of ethereum and intends to stay that way will take that even further. There will probably continue to be forks of ethereum, just like there have always been. Anyone can fork an open source codebase and maintain it, anyone can use the software to spin up a consensus network, anyone can use prior state as a starting point. Chain forks are a social thing not a technical thing, game theory not withstanding, that's how nakamoto consensus and FOSS is supposed to work.
Legitimacy is determined in the eyes of the userbase, in which software they decide to use and what ledger they regard as the canonical history of the network, i.e. what blockchain they sync. Whichever "Ethereum" the majority of users use is the "Ethereum."
Everyone doesn't have to agree. There will be miners that attempt a fork to keep their position. There is nothing wrong with this, it's how this stuff is supposed to work. But the fact that PoS has been on the roadmap since the beginning takes away legitimacy from any claim that the PoS transition is illegitimate. The fact that Ethereum Classic exists and is a PoW version of ethereum and intends to stay that way will take that even further. There will probably continue to be forks of ethereum, just like there have always been. Anyone can fork an open source codebase and maintain it, anyone can use the software to spin up a consensus network, anyone can use prior state as a starting point. Chain forks are a social thing not a technical thing, game theory not withstanding, that's how nakamoto consensus and FOSS is supposed to work.