> isn't this problem largely solved by other products

No? I don’t want to use my speakers as output for all the audio from any given device, only the stream I want. So, the audio system needs to be capable of managing the stream itself, independent of the device I use to select/start/stop the stream. I don’t know of anything other than Sonos that manages that well.

(Never mind the sibling comments point about syncing multiple wireless speakers to play the same audio without perceptible lag)