I have no idea why, I’m impacted by this as well. If you have access to the desktop app you should be able to get your library back on Sonos through smb. You can’t search, navigating is slow and frustrating, your library may randomly appear or disappear in the app etc. in the new app but it’s better than nothing (maybe)

There’s also Plex. It will cost upfront and you need hardware to run it on. 

I had a good go at them over the phone earlier stating my/our case. She mentioned the desktop version (which will be withdrawn soon). I don’t want this as the iOS app suits me. I have invested significantly in both hardware and music and wish to continue with this.