you do realize that you could (have?) import all google music files to youtube music. Then you have a tab saying "own content" or so and all mp3s are still there and you could listen to them via yt music the same as you could before
And more generally, while I don't necessarily mind a rebrand (although I have to admit that this being Google, I have my suspicions that this will be handled the same way they handle all their messaging services) and I can be forgiving for a v1, this is still so far below play music that I don't really see myself using it.