The problem at this point isn't whether chrome will disable it... It can be added back, probably fairly easily as google intents to keep it around for "enterprise" anyway, so it's probably just gonna end up changing/removing one if-statement. If not, then the code already exists and has to be merged back in.
The problem is distribution of the actual extensions, as Vivaldi kinda pointed out, as the other browsers mostly rely on the Chrome Extension store. If Google really decides to pull the plug here they could start rejecting extensions using the old, removed-from-Chrome APIs. And then what? Each browser such as Vivaldi, Brave, Opera has to implement their own app store just to host the old-API ad-blockers. And implementing such a store is a major feat. And the walled garden strikes again.
Vivaldi already indicated they might create a "limited" store, meaning not a real store open to the public, but one where they list hand-picked old-API extensions such as ublock origin. Of course, innovation is still stiffed as you in such a "chrome-store + limited vendor store" scenario cannot just develop new extensions using the old-API because you'd have no chance of getting listed in the chrome store, and essentially no chance in the hand-picked limited vendor ones.