I think the concern here is that, as a practical matter, vendors wouldn't be free to make arbitrary changes within their own forks; some parts of the system would have to remain interoperable. And technical decisions regarding what those parts should do and how they should work would still have to be made somehow, and it's not clear that "open source project" type governance would be better than the W3C process that we have today, especially when it comes to interfacing with governments. Good blog post about this: https://www.mnot.net/blog/2022/06/22/chromium-only