It’s a question of trust. Google will select the certificates they trust for the services they provide, and the entities that own those certificates will decide what do to with them. If they trust a certificate from Mozilla, and Mozilla agrees to make that certificate open to everyone for instance, than Google’s only choice is to stop trusting it. But if Mozilla decides that is the certificate Firefox will use, than Google has to choose kicking off Firefox as well as other third party apps. Same with Microsoft and Apple, but I think Mozilla is more likely to oppose this kind of standard rather than try to reach some kind of agreement with Google.
The other way that this could play out every browser dev makes some kind of arrangement. Very unstable when we are talking about competitors.
At the end of the day, it requires a level of co-operation with the browser developers and internet service providers that I don’t think a lot of people will go for, for various reasons. Especially not regulators. I guess I am just more optimistic about the open internet.