Market Concentration Barely Changes Model Collapse: Pushing One Model to 90% Share Moves Five-Generation Endpoints a Few Percent

arXiv 2609.11146·medium signalXBlueskyLinkedInCopy link

arXiv 2609.11146 (submitted 10 Sep 2026) tests the assumption behind multi-model collapse studies, which usually split the market evenly while real generative AI is an oligopoly. Thirteen open 1-4B models form ecosystems of 3 to 13 players plus an injected probe that pushes the top share to 90%; each generation mixes every model's output into a shared pool by market share and retrains all models from clean base weights, for five generations. Making the split more unequal barely changes collapse speed, and the share and identity knobs shift five-generation endpoints by only a few percent of the drift common to all arms, so the ecosystems collapse to nearly the same place even at extreme share paired with the strongest injected bias.