The thing about fork bombs that it’s not particular process which takes up all the resources, they’re all doing nothing in a minimal amount of space. You could say “ok this group of processes is using a lot of resources” and kill it but then you’re probably going to take down the whole user session as the starting point is not trivial to establish. Though I guess you could just kill all shells connected to the fork morass, won’t fix the general case but it’s a start. OTOH I don’t think kernel devs are keen on special-case solutions.