The point is there usually isn't a premium service for sites blocked by UBO. So doing just YT Premium with UBO for other sites would only entrench YT and deny creators on other services a significant source of income.

Maybe if advertisements don't work for them, and the subscriptions work for YouTube, they'll roll out subscriptions too. But to be honest, I just hate advertisements, and so I'll block them, and I'll also subscribe where I can because I want to give back something too.