The main part of the browser is its main developer. In this case it is Google. If Google decides to add QUIC support to the browser it will appear in all of the clones too, because they are not standalone. If Google decides to move to the Manifest V3 then all closes will move to it too. Whatever Google decides to implement deep in the Chrome browser, other will have no choice but to accept.
Yes, if they will keep this promise it will be great. It is a big effort for almost no profit, so I guess then Brave could be counted as a separate browser.
It would probably depend on the extension authors too, since they would need to support two different versions, one of which for the Brave only, and another for all Chrome variations including Chrome itself.
