What a shame. What's the point even? I'm not going subscribe to YouTube premium anyway, and even less install and use the YouTube app. What happens with this move is just that I will just use YouTube less. I believe that's the case for most people who chose to use YouTube in a browser precisely for background playback.
As an example: knowing that I won't be able to keep the sound playing for the 5 minutes in between two buses when I need to walk and pay attention, I'll probably just launch a podcast from the beginning of my hour of transportation so that I'm not interrupted. For these five minutes, they loose me for almost an hour.