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.