The default case when implementing a change should obviously be to make changes with as minimal a blast radius as is reasonable. This is basic software engineering and current LLMs fail it. LLMs also don't have a good idea of what is "reasonable" and a human judgement call is needed.

The case where a (sub)system needs a complete rewrite to admit a feature without incurring too much technical debt should be the exception. When exactly to make that exception is something that clearly currently requires a human judgement call, as models aren't yet nearly smart enough to make such calls.