I was thinking about this as a bull case for human developers. Seems if you're worried enough to do this you're not going to have LLMs write the new code.
Large companies already maintain a clone of their packages. Very large ones actually bundle their own build system (Google Bazil, AWS Brazil). If you want to update a package, you have to fetch the sources and update the internal repository. It slows down the opportunities for a supply chain attack down to a crawl.
The flip side of that is now you're running old software and CVEs get published all the time. Threat actors actively scan the internet looking for software that's vulnerable to new CVEs.