You would use ranking algorithms meaning you should crawl, scrape and analyse YouTube videos which would make YouTube probably unhappy.
I don't get that. It should have the exact same incentive to down-vote, unless you think Google no longer collects or uses that information as opposed to just hiding it.
The incentive for users/viewers to downvote something has always been to give other viewers a heads up. This doesn’t work anymore
Previously, disliking would increment the global "dislike" counter and users got immediate visual feedback that their dislike was counted, even if that state may take minutes for Google to reconcile and store.
Now, clicking the dislike doesn't do much and users have no indication whether others disliked the content too.