Not to disagree with the request, as it something I want and would use as well, but it’s probably not quite as simple as you think.  The current Sonos app is just a controller, not a music player.  Sonos would need to add a music player feature to the app, or a new app.  Perhaps the development for this feature is pretty straightforward, but it would mean that the player would need to be play at different qualities depending on the capabilities of the phone or tablet.  Something currently not a concern for Sonos radio.  As well, licensing of the music may be significantly different  when you change the scope of where Sonos radio can be listened to...particular for the non-custom stations.

Regarding protentional Sonos headphones, I would guess they would operate like Sonos Move.  In WiFi mode, you can access Sonos radio, but in BT mode, you have the same access as any other bluetooth headphones or speakers.

Danny totally agree with what you say regarding the Sonos S1 and S2 app and Licences (tricky ground look at tunein ending up in the high courts) the app IMO  is very sophisticated and versatile and has been IMO developed well (so well in fact it blocks every VPN I don’t use it for piracy but security honest) 

All I want is the ability like yourself to listen to Sonos Radio and all the SONOS curated Radio Stations not necessary in HD. I could listen to Thom Yorke all day and night but I think some of his playlist selections may drive the missus mad with its constant droning sound lol . 
 

word on the street though is 2021 could well be a launch of Sonos headphones as well as the Chemical Brothers  coming to Sonos Radio in March so I think I might re-invest my £7.99 back to Sonos 

stay safe 

M