Modern player is not only about UIUX but also which music storage it supports, DLNA, Plex, Jellyfin, that’s the main feature users will be checking. If it’s just AppleMusic player.
It’s not like streaming platforms are gingerly implementing open and common standards for music streaming. Winamp had support for .m3u and streaming back in ‘04, but we’ve moved on from that.
AKA the limited resource of developer-hours
Sure, but each connected backend magnifies value to end-user. To say any one client is worth devoting to just a single backend is crazy.
A music player is very much a dumb pipe (or "toaster" as you call it). There's nothing special about Apple Music that makes it a backend worth devoting an entire client to.
Well, considering it's an Apple Music frontend… that's really the point, isn't it?