In proof of stake, that could be a problem (though there are mechanisms people claim can handle it). But in a proof of work currency, having majority consensus

In proof of stake, that could be a problem (though there are mechanisms people claim can handle it). But in a proof of work currency, having majority consensus would mean they'd have to have the majority of miners - and even then they have to follow the rules because users' nodes will just reject invalid blocks. And in the worst of cases, people can fork away, and if they change the PoW algorithm while doing so - they'll also render the malicious actor's ASICs useless. I'm not saying this is fool proof, but the innovation of Bitcoin is precisely the system which is designed to resist these things (again, it's still very hard).