It is in YouTube’s interest that the videos uploaded in it are popular. What makes videos less popular? Negative notes, largely. That is why, though of course no one will admit it officially, YT plans on removing thumbs down in an unforseen future. Google also has other, similiar ideas: remove the thumb down by default but leave option of seeing it, leave the thumbs down but hiding the counter (useless) and others.
What do you think? Majority of the Internet seems to believe that this would be simply a cheap way to make YouTube’s life easier at the expense of viewer’s abiity to show their disapproval.