Anyone can "fork" it by making their own build. Which will quickly become worse than useless as it falls behind security patches. A secure fork is something that is easy to start, very very hard to maintain.

The possibility of forks does little to prevent a Chromium monoculture from embracing, extending, and extinguishing the web. (Something like Brave, with an independent revenue model, does help some with the bad maintainer problem. But it doesn't help with the constant stream of everyday bugs in the underlying engine, ones that would get set in stone with a monoculture.)