The assumption that Chromium (& Webkit) is controlled soley by Google, is outdated.
If you think Microsoft would have made a strategic shift like basing Edge on Chromium without ensuring they had a seat at the table, you would be very mistaken - and they did not.
Microsoft, Google, Brave, and Samsung all contribute to the core, Microsoft very significantly.
You're arguing "sole" control which is a Strawman and doesn't really address anyone's actual concerns. A better discussion is: Can Google make unilateral decisions on Chromium's direction/support/features in any circumstances?
What I'm about to say may upset people but if Chromium's governance model was good enough I could see Firefox joining it and turning Firefox into another Chromium based browser.
The problem with hegemony is governance and control, it isn't technological. Purely on a technical level hegemony is a good thing. But who controls the hegemony? If it is Google or any corp that is really, horrible.