It's also now a lot easier to fork an open source project and tweak the last 10% so it works exactly as you want.

Exactly. Whilst I can see the problem with vibe-coded "contribution" that lower the signal/noise ratio on big OSS project, it's also "liberating" in the sense that forking becomes much more viable now. If previously it took time to dive into a project to tweak it to your needs, it's now trivial.

So in many senses AI is democratising open-source.