How could advances in programming languages still happen when nobody is writing code anymore? You think we will just ask the AI to propose improvements, then evaluate them, and if they are good ask the AI to make training samples for the next AI?

Maybe, but I don't think it's that easy.

If we were to reach a stage where humans don't write code any more, would there even be a need to have advances in programming languages? Maybe what we currently have would be good enough.

I don't know what future we're looking at. I work in aerospace, and being around more safety-critical software, I find it hard to fathom just giving up software development to non-deterministic AI tools. But who knows? I still foresee humans being involved, but in what capacity? Planning and testing, but not coding? Why? I've never really seen coding being the bottleneck in aerospace anyway; code is written more slowly here than in many other industries due to protocols, checks and balances. I can see AI-assisted programming being a potentially splendid idea, but I'm not sold on AI replacing humans. Some seem to be determined to get there, though.