I strongly suspect they're gonna drop support as soon as the first bigger merge issue happens along with a heartfelt blog that "they did they everything to support it, but it was just too much for the resources available to them"
I doubt it's gonna take more then 1-2 years (December 2027) for this to happen, but we will see.
I expect Brave to easily support it until then and then drop it very quickly as you described.
That is, if those companies choose.
If even 80% of them wanted to fork? Not a biggie. And they could still cherry pick commits from the alt fork.
It might be easier to maintain than an actual extension interface with hooks thought the code.
The reason why ublock origin is so powerful is because it works with the DOM/not at the network level and can use heuristics to determine wherever something is a advertisement or not.
"Excluding merges, 684 authors have pushed 3,139 commits to main and 3,866 commits to all branches. On main, 14,924 files have changed and there have been 740,516 additions and 172,682 deletions."
That's stats from last week. Last year Google apparently was responsible for about 95% of contributions. Other than Microsoft (which has the same bad incentives as Google) none of the alt-chromium browser companies has like, 5% of the engineers to maintain a real alternative
Opera has pinch-zoom text-reflow in a chromium backend, and that seems to be substantial, and yet it is (on purpose) kept out of mainline chrome. So they do loads of tracking/merging too.
The scope of work to do a few small features on top of chrome wouldn't be a biggie, compared to the entire project.
So even when they have to say farewell to Manifest v2 it really doesn't matter, at least in case of privacy (and for some medical) protection.
And that's a lot of work for a multi million LOC project, unless the architecture is specifically made to support such extensions... which isn't the case here.
And freezing your merges indefinitely isn't really viable either for a browser
That's been the default assumption of pretty much everyone anyway.
If not today maybe soon...
Being independent of google requires actually doing the work and not just copying google.