Regarding LLM's performances on brownfield projects, I thought of Naur's "Programming as Theory Building". He explains an example of a compiler project that is taken over by a team without guidance from the original developers:

> "at [the] later stage the original powerful structure was still visible, but made entirely ineffective by amorphous additions of many different kinds"

Maybe a way of phrasing it is that accumulating a lot of "code quality capital" gives you a lot more leverage over technical debt, but eventually it does catch up.