Going by the system card at: https://openai.com/index/gpt-5-system-card/ > GPT‑5 is a unified system . . . OK > . . . with a smart and fast model that answers most questions, a deeper reasoning model for harder problems, and a real-time router that quickly decides which model to use based on conversation type, complexity, tool needs, and explicit intent (for example, if you say “think hard about this” in the prompt).…
Too expensive maybe, or just not effective anymore as they used up any available training data. New data is generated slowly, and is massively poisoned with AI generated data, so it might be useless.
People evaluate dataset quality over time. There's no evidence that datasets from 2022 onwards perform any worse than ones from before 2022. There is some weak evidence of an opposite effect, causes unknown.
It's easy to make "model collapse" happen in lab conditions - but in real world circumstances, it fails to materialize.