You're ignoring the information about the source. It's not about a heuristic on text of unknown provenance -- it's about publishers that exert a certain level

You're ignoring the information about the source. It's not about a heuristic on text of unknown provenance -- it's about publishers that exert a certain level of editorial control and quality verification. Or social reputation mechanisms that achieve the same. That's what is preventing your "model collapse". Reputations of provenance. Not pure-text heuristics.