Right, but AI could change the ratio of greenfield vs brownfield then (« I’ll be faster if I rewrite this part from scratch »)

I struggle to wrap my head around how this would work (and how AI can be used to maintain and refine software in general). Brownfield code got brown by being useful and solving a real problem, and doing it well enough to be maintained. So the AI approach is to throwaway the code that's proved its usefulness? I just don't get it.