The whole idea around proof-of-work based chains isn't immutability, it's that the longest chain always wins. If the majority of CPU power 'votes' for a particular chain that chain sits as the de-facto chain. For most chains this means >50% wins, as requiring anything higher for consensus increases the potential for network deadlock.

With that, are there any trust algorithms that aren't susceptible to 51% attacks? Ideally power (stake or hash in this case) is distributed amongst actors as much as possible. Cryptocurrency doesn't take us from centralized -> decentralized currency: it moves us from almost wholly centralized currency to more decentralized currency. The move is welcome but idealistically seeing the options as completely centralized or completely decentralized seems a bit misguided to me.