I think most people that traditionally used iTunes didn’t keep other copies somewhere else, since it was meant to be the music manager for all music, so if it screws up their library they lose their files.

The uploading and syncing local files was (is) already a feature of iTunes Match. Apple Music just expands it to allow it for music they don’t own, however people have had it take their files and relabel them as Apple Music files and then lock them out if they cancel their subscription.

The downside is combining my local music management with their streaming service, I’d rather they were entirely separate with the option of playing local files, as Spotify does. The option to upload files would be fine.