To be clear, they aren't removing the button, or a way for users to register their dislike or track that they already disliked a video (I assume), just the count of how many other people disliked it. As a matter of engagement, I'm not sure how much that statistic brings to the table. I agree it might make filtering out crap content somewhat harder, but I'm not sure how good it ever was for that. It might be that the vast majority of the time it indicated anything to the viewer it indicated that some campaign was underway, which generally is counterproductive as a viewer.

Personally, I upvote if I liked something, and I don't indicate anything if I didn't find it all that interesting or useful. Rarely if ever have I disliked something that youtube has recommended to me. The few times I have it's probably been because of some outside source noting it because it's objectionable. How useful of a signal is that to youtube or the creator? In that case it indicates what people that don't care about their content and aren't likely to watch any more of it think. That seems a poor source of feedback (and if we're talking about hat being useful to indicate to them that they shouldn't have done whatever they did, that's obviously served already by many other mechanisms, including comments).