How are they going to be adding uBlock Origin to Chromium going forward if manifest v2 gets completely deprecated/removed entirely?
Nothing. It will be a huge burden for them to maintain all the removed code. Their only choice is to integrate brave's adblocker.
AFAIK some of the other chromium forks (brave and/or edge?) were committed to backporting manifest v2 (or more specifically the webRequestBlocking API) for future chromium versions.