I get where you’re coming from. I suspect that much of that flexibility was never intended. After all, OS X started as a workstation OS for a different market than Apple entirely.

I won’t address your whole comment but I feel compelled to point out that Music is iTunes. The Music app was just a name change, and, confusingly, Apple Music (the service) is an entirely different entity than Music (the app). Your own local music files (either your own MP3s or iTunes Store purchases) are still accessed in the same iTunes Media Library folder as they always were.

Music was just a rebrand, and it spun out podcast, audiobook, and iPhone/iPod sync. But the rest of it is 100% iTunes.