You can fork it and basically freeze it at manifest-v2.

The problem is, all the big tech sumbitches, their buddies and all the companies who want a corporate website that Just Works [tm] will support Google's new shit, and your privacy-respecting fork will slowly deprecate and stop working right, because you don't have the resources to mirror new features in Google's official browser. And of course, ordinary internet users with stick to Google's version because they need a browser that works.

Chicken and egg... In fact, that's exactly what's happening to Firefox and why it's sliding into irrelevance: Google is simply too massive and too monopolistic to resist for very long. Mozilla has had hundreds of millions to throw at trying and even they are on the verge of losing the battle completely.