The LLM can a priori test on all possible software and hardware environments, test all possible edge cases for deployment, get feedback from millions of eyes on the project explicitly or implicitly via bug reports and usage, find good general case use features given the massive amounts of data gathered through the community of where the project needs to go next, etc?

Even in a world with pure LLM coding, it's more likely that LLMs maintain an open source place for other LLMs to contribute to.

You're forgetting that code isn't just a technical problem (well, even if it was, that would be a wild claim that goes against all hardness results known to humans given the limits of a priori reasoning...)