A browser monoculture allows one entity to dictate how the web works. Even if you can open a pull request against Chromium, that doesn't mean it will get accepted without the approval of Google.
Right now, backwards compatibility is protected by competing browsers. If one breaks backwards compatibility too often, it risks becoming known for having sites not work on it.
In a monoculture, Google could take aggressive moves to prevent ad-blocking. In a monoculture, Google could push more ad focused features like FloC. Google could integrate more ways to allow browser fingerprint (not saying they would, but there would be no recourse).
With competition Google knows that people could balk at any point so they must balance their interests with their users' interests. In a monoculture, they wouldn't have to.
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.)
We're already there. 4% Firefox market share may as well be zero. What is the current recourse for Chrome pushing a monoculture? I don't think it matters today what Firefox does or objects to.
Right now, backwards compatibility is protected by competing browsers. If one breaks backwards compatibility too often, it risks becoming known for having sites not work on it.
In a monoculture, Google could take aggressive moves to prevent ad-blocking. In a monoculture, Google could push more ad focused features like FloC. Google could integrate more ways to allow browser fingerprint (not saying they would, but there would be no recourse).
With competition Google knows that people could balk at any point so they must balance their interests with their users' interests. In a monoculture, they wouldn't have to.