Why exactly would there be less content with YouTube Red if the "partners" didn't switch over? I guess I also don't understand why the TOS of the partner program enable them to unlist the video, but not sign them up for the program.
As the top comment (and others) point out, Youtube can't show paying customers ad-filled videos, so they would have to avoid showing those videos altogether. At that point they'd be asking users to pay for a service that has fewer videos than the free tier.
