Google may discourage forks, but there's nothing they can do to prevent one, and forking Chromium is no more difficult than forking any other project its size.
... or any contribution that doesn’t fit their ideals.
That's one of an OSS project maintainer's more important jobs. I used to maintain some decent-sized open source projects, and part of my job was to say "no" when someone wanted to add something I didn't think belonged (for whatever reason).
Forking Chrome would be like fighting against Embrace, Extend, Extinguish after you’ve already lost (which we have with Chrome, compared to what KHTML used to be like)
Regardless, though, there's nothing that says the hypothetical developers of a Chromium fork couldn't devote some time to merging reasonable fixes & new features from upstream.
Google even openly discourages forks, or any contribution that doesn’t fit their ideals.
It’s only truly open if development is controlled by a democratic community, or if it’s easily possible to be forked and competed with.
Which isn’t the case.
And for users open development is a lot more important than open source: Being able to get their own changes into the browser, ensuring the browser is made by the users, for the users.