Code quality will impact the effectiveness of ai. Less code to read and change in subsequent changes is still useful. There was a while where I became more of a paper architect and stopped coding for a while and I realized I wasn't able to do sufficient code reviews anymore because I lacked context. I went back into the code at some point and realized the mess my team was making and spent a long while cleaning it up. This improved the productivity of everyone involved. I expect AI to fall into a similar predicament. Without first hand knowledge of the implementation details we won't know about the problems we need to tell the AI to address. There are also many systems which are constrained in terms of memory and compute and more code likely puts you up against those limits.
I don't disagree that code quality is currently more important than it's ever been (to get the most out of the tools). I expect that quality will increase though as people refine either training or instructions. I was able to get much better (well factored, aligned to business logic) output that I'm generally happy-ish with a couple months ago with some coding guidelines I wrote. It's possible that newer models don't even need that, but they work well enough with it that I haven't touched those instructions since.