Even if maintaining Manifest V2 is too much work after upstream drops it that doesn't necessarily mean that they just have to accept Manifest V3 as shipped in upstream.
From what I've read there are two main problems with Manifest V3 when it comes to ad blocking.
Problem #1. The webRequestBlocking permission is gone:
> Note: As of Manifest V3, the "webRequestBlocking" permission is no longer available for most extensions. Consider "declarativeNetRequest", which enables use the declarativeNetRequest API. Aside from "webRequestBlocking", the webRequest API is unchanged and available for normal use. Policy installed extensions can continue to use "webRequestBlocking".
That does not necessarily mean good ad blocking must break. Most people find the ad blockers on Safari to be fine, and Safari does not have that kind of blocking. It has long required the declarative rule kind of blocking.
Some brief searching suggests that the rules for Chrome's declarative blocking are actually more powerful than Safari's.
That brings us to the second Manifest V3 problem.
Problem #2. The declarative rules only allows something like 30k rules. Apple found that such a low limit is not sufficient and raised it to 150k.
I'd guess that maintaining a fork that just raises the limit in Chrome would not be a problem for the Chromium-based third party browsers.
The big question would be whether the current 30k limit is there for technical reasons or for marketing reasons like not wanting ad blocking to be too effective. If it is a technical limit the forkers might also have to rewrite the rule processing engine which would likely be more than they would want to deal with.
[1] https://developer.chrome.com/docs/extensions/reference/api/w...