But YouTube is already using the dislikes to push content down, it's just not showing you the count (nor does it show you the many-other engagement signals it uses for its rankings and recommendations).

Personally, I see the removal of visible dislikes as a net positive. They were used to rain down negativity and meanness on well-intentioned videos. I think YouTube is now a happier place when a kid's crappy singing video gets quietly ranked down, versus being shown to have 34 classmates (or 34,000 strangers) that hated it.