If anything, it's more data to track if you're paying. Plus, you can't use it in a private browsing window, since it's tied to your account.

As they say, if you ain't paying, you are the product. And if you are paying, you are a premium product.

That and make the like/dislike ratio visible again. They really have no reason to make you sit through a scam video long enough to be served an ad when you’re paying.

There are extensions to bring back dislike button, makes a world of a difference

If they stopped tracking you, they wouldn't be able to show you targeted ads, which would make the ad revenue go way down. In other words, it's not gonna happen.

But you’re not getting ads when being subscribed to premium, right?

True, but you still get ads in search and other places, so tracking your viewing preferences on youtube is very valueable.