> the clean modeled structure we want to see is for humans to maintain and extend, but the AI doesn't need this.

This does not match my experience. I do a lot of AI-assisted coding at this point, and what I've seen is that when the AI is asked to extend or modify existing code, it does a much better job on clean, well-structured and well-abstracted code.

I think the reason is simple, and tracks for humans as well: well-structured code is simply easier to understand and reason about, and takes a smaller amount of working-set memory. Even as LLMs get better with coding, I expect that they would converge on the same conclusion, namely that good structure + good abstractions make for code that is more efficient to work with.

Yeah I have had claude take over multiple internal (human written) projects that were in a dire state and spent a week just completely refactoring them and adding exhaustive tests before doing any new features. It's worth starting from a clean slate.