YouTube music isn't really a different service rather than a different YouTube app. Under the hood YouTube music is just YouTube with a music player UI. Taking it away wouldn't really lower the cost much.
I find it hard to justify paying for 2 music streaming services, so I cancelled Apple Music, because I'm paying for YouTube Music through Premium. However, I don't like it, so I'm back to manually managing a local music library in Apple's Music app. This is probably a better long-term approach than renting access to a music library on a monthly basis.
The whole "YouTube music free!" is just marketing and a music focused app wrapped on YouTube.
YouTube premium without YouTube music would be pretty much the same cost.
YouTube premium would not be any cheaper without YouTube music. It's a marketing gimmick.