Of course market share has to be part of the discussion. But the way in which you used it seems tautological. Also, market share affects alternative browser engines just as much as Chromium based browsers.
>The original argument was that Google wouldn’t control the web in a Chromium monoculture because anybody can just fork it.
That wasn't my argument though. I don't know if Google wouldn't control the web in a Chromium monoculture. It very well might. My argument was that the IE era does not serve as a valid historical precedent because the fact that Chromium is open source changes the situation in very significant ways.
>A Chromium fork can only sever itself from Google’s control if it is not taking patches from upstream (ie, Google)
I don't think Chromium based browsers can completely extricate themselves from Google's control. But this is not a black and white question. Alternative browser engines cannot do that either.
You make a good point that web APIs are a better test for Google's control than ad tech. But this kind of control affects independent browser engines just as much as Chromium based ones. If Chrome doesn't implement a particular web API then the API is dead in the water. That's where market share matters.