They can, but that means dealing with the overhead that comes with maintaining a divergent and ever increasingly different code base from the forking point.
Every single browser except for Safari and Firechrome today is based off Chromium because devs are either lazy or incapable of writing and maintaining their own browser engines.
So can browsers forked from Chromium ignore this and keep MV2? Sure. Will they? Hell no.
Google pay other browser vendors significant amounts to be the default search engine. If any browser were too far ahead of Chrome on blocking advertising, wouldn't Google care substantially less about being the default search engine on that browser? Ad blockers switch off the primary revenue stream for browsers & websites alike.
