When it's your own codebase that you know intimately, you obviously don't want it polluted, and you want to continue to understand everything that's there. For rewriting things you understand perfectly, or porting code to different platforms, the LLMs truly are a force multiplier. But that's so different from the way they are used on new projects. Letting them make design decisions is the problem. To make design decisions, you have to understand the system as a whole.