Browser vendors monetize your data, that is their main trade. Having lots of users doesn't matter if you don't control them in some way, such as decide who gets to be trusted as a CA. The browsers are supporting the current tech hegemony for CA's, for some reason Americans are much more "trustworthy" than people from anywhere else, do you really believe that? Why should the people in Europe be forced to trust shady American companies like Google or Microsoft who are or likely will get compromised by the American state? It makes sense for them to regulate this so that they can have European options.

> it's their stock in trade

Their stock in trade is being easier to use and more feature-rich than the competition. Trust doesn't factor into it, in the sense that trust in a browser does not come from track record: It's all marketing, all the way down. Thus, if they screw up it does not matter. This point is easy to see because they frequently do screw up and it hasn't had any impact so far. There's no reason to assume it will have one some day.

Where exactly do people move if they only chose between Firefox and Chrome, there is not enough competition in the browser market