Youtube have always provided a relatively low bitrate quality 1080p video to everyone.
Now they have a Premium option for a higher bitrate.
Existing user without Premium are NOT losing out, quality remains the same as it always has.
And this one works for 1080p 30fps video, not higher or lower bitrate. And not for 4k video that's downsampled to 1080p
You're right.
But on the other hand. Previously, free tier YouTube users had all available resolutions, quality settings, etc. Available.
Now they are changing that policy.
It could be the thin end of a wedge, and as time goes on, more and more features (either existing ones, or new features), get put into ONLY the premium service.
To many people (I think) and some countries, the £12.99 per Month, is not going to happen, as it is too expensive, compared to their available resources.
Edit: But if I partially answer my own post. It is a bit like everything else, e.g. super fast internet connections, can cost a lot of money per month, hence more well off companies/people, tend to get them.
So the concept that paying more money gets better/faster/more stuff, is well established. So the YouTube premium is just another example of the same concept. I.e. You pay more, you get something better.