> planned architecture or grown organically
That sounds like a hard or, as if ten years of development of a large codebase was entirely known up-front with not a single change to the structure over time that happened as a result of some new information.
"We build our computers the way we build our cities—over time, without a plan, on top of ruins." -- Ellen Ullman
The important thing about a codebase wasn't ever really size or age, but whether it was a planned architecture or grown organically. The same is true post-LLM. Want to put AI in charge of tool-smithing inconsequential little widgets that are blocking you? Fine. Want to put AI in charge of deciding your overall approach and structure? Maybe fine. Worst of all is to put the AI in charge of the former, only to find later that you handed over architectural decisions at some point and without really intending to.