All pre-existing monkeys get grandfathered in to any sort of new fork
they would be grandfathers without any descendants though. no more making new monkeys.
unless you're including some kind of rollback to the pre-Taproot Wizards days, which makes things super complicated and let's face it, isn't gonna happen.
i would think that the bitcoin cash hard fork was way more complicated than just rolling back bitcoin to some previous state...