I understand your point, but my take is that when we talk about AI and its impact, we're talking about the entire system: the model, and what is buildable with the model. To me, the gains available from doing innovative stuff w/ what we're colloquially calling "UI" exceeds, by a bunch, what the next model will unlock. But perhaps the main issue is that whatever this amazing UI might provide, it's not protectable in the way the model is. So maybe that's the answer.
If I were OpenAI, I would 95% of resources on ChatGPT5, and 5% into UX.
Once the dust settles, if humanity still exists, and human customers are still economically relevant, AI companies will shift more resources to UX.