One of the most powerful things about a blockchain is the fact that anyone can run a node on their computer and verify that the chain is correct. Even if 95% of the nodes running the chain consensus (PoW, PoS...) all immediately agreed to change the rules, and started producing blocks according to the new rules, everyone running a fully-verifying node would refuse to accept the chain. The stakers who are not part of such a cabal would automatically converge on, and continue building, a chain that continues to follow the old rules, and fully-verifying users would follow that chain.