Major exchanges going down has historically caused crashes, but temporary ones. There are two things that would truly destroy crypto as we currently know it: a technological problem being found in the protocol of at least Bitcoin if not others, and a general understanding and acceptance by the public of the complete uselessness of crypto for anything other than essentially scamming people.

A worm that stole a few billion dollars' worth of crypto from all over the Internet has the potential to hit both of those points, I suppose.