I'm not a cryptocurrency person, but my understanding is that that's happened before: ETH effectively rolled back the DAO hack[1] in 2016. The end result was the "Ethereum Classic" split, which continues to this day. There are all kinds of financial ramifications to this kind of split, nearly all of which (to the best of my knowledge) remain unsolved.
The problem that you've correctly observed gets to the very root of why cryptocurrencies are a farce: if participants need the confidence of an ultimate human democratic process, you might as well kick the immutable public ledger to the curb, skip the tire burning, and use the financial system we already have.
Yes, Ethereum Classic is still traded (although I have no idea how thinly). That's exactly why it's a problem: every account pre-fork now exists on two blockchains, and it's not clear what the tax, contractual, legal, etc. ramifications are of essentially doubling everybody's money. The nature of the blockchain also means that forks are destructive against innocent transactions: users doing "business as normal" who have the misfortune of being included in or after the rollback block have to re-coordinate all of their work.
The problem that you've correctly observed gets to the very root of why cryptocurrencies are a farce: if participants need the confidence of an ultimate human democratic process, you might as well kick the immutable public ledger to the curb, skip the tire burning, and use the financial system we already have.
[1]: https://en.wikipedia.org/wiki/The_DAO_(organization)