The upload-and-stream-your-own-music feature, as handled by the Mac desktop Apple Music app, seems to be 90% bug and 10% working. I can’t imagine a rewrite being worse than the status quo.
Considering the state of every recently made/"remade" first party Apple app I cannot imagine how horrible the Music app would be after they got done with it.
At some point when migrating from one Mac to another, it "forgot" which songs were actually mine. It's all Apple Music now. I have songs that the application _knows_ were added to my library in 2003, but for which it steadfastly maintains they're Apple Music downloads. Worse: some songs have been replaced with other recordings. Other are "unavailable" for unexplained reasons.