Miners follow the longest valid chain. A 51% hashrate cannot unilaterally alter the protocol rules or invalidate existing coins; the network's full nodes maintain the true consensus.

The 51% of hashrate can alter the protocol rules and invalidate existing coins, if they do it via a soft fork. A simple example: They decide to ban OP_RETURNs greater than 83 bytes, and they decide to mine only on top of blocks that also follow the same rules. As they have 51%+ of hashrate, their chain will be the longest chain. Nobody will ever be able to have a large OP_RETURN again. The remaining hashrate will either have to give up, or follow the new rules. The only other alternative is to have an opposing softfork, which requires large OP_RETURNs. Enforcing this at one activation height is sufficient