This doesn't mean it is a "success" in the context above. The population living there might have lost most of its previous technological abilities and stabilized in a permanent "collapsed" state. There is no reason that humans have to go extinct when they lose their cultural achievements, after all, other animals survive without having those in the first place.

Humans would only go extinct if they lived in an environment which required a certain minimum of cultural ability, e.g. making clothes in areas with a colder climate, as humans don't have sufficient fur.