Forking Chromium won’t help you if Chrome’s market share is so overwhelmingly large that it can basically force its ideas down the web’s throat.

If you're worried about there being a browser monoculture, being able to fork is useful but not sufficient. For example, suppose Microsoft's new browser gets a large enough share that they (along with Firefox) can veto some of Chrome's changes? Since they have enough technical competence and resources to maintain a branch, upstream doesn't control what they release; they can take changes or not. And they also have ways to gain significant user share.

We are probably past the point where a scrappy new open source browser could gain enough developer talent and user share to be relevant, though. Particularly since it can't be done without mixing in some proprietary code for DRM.

So there may be competition, and a significant chunk of open source code will be involved, but it won't satisfy the folks who want a pure open source browser.