rewriting code is the easy part

it's the testing in real world usage to shake out all the edge cases and bugs that's hard and that you can't just write unit tests for

I think we can all agree that the task as set forth in the PR puff piece is impossible. That said however, current AI tools are amazing for cleaning up and reanimating old codebases. I've been using Claude on a very large Java protocol proxy product and it's been able to do amazingly useful things for me. I originally wrote much of the code but Claude now knows much more about it than I do today, because people forget stuff over decades. It will sometimes remind me "as you noted in November 2016 this feature was designed to...", based on its analysis of the code and git commit messages.