This seems to imply that everyone else's stake in the network would become worthless too, though. What stops a particularly motivated billionaire from buying 51% compute for a month or so to destroy a cryptocurrency for good? They waste that money but any competitor of theirs (in business, etc) who had wealth tied up in that currency can kiss it goodbye. If a business was built around that currency it's dead now.

The 'diminish trust in the network' threat really only discourages an existing good actor (invested in the network) from turning into a bad actor.

If I've been mining a cryptocurrency for years and I decide I want to move on to mining something else, what stops me from using 51% to destroy the currency before I move on to mining something else? If I moved my currency holdings out (or have been liquidating them regularly, as any sensible miner might), what's to lose?