I know it has been raised before (some years ago) and there were some good arguments against doing it. But as the market is evolving this topic probably is regaining importance again.
I just found myself in the position to buy a thread border router and now had to add the first Apple homepod mini to my Sonos mix. I would have gone for a Sonos speaker, or even better would have appreciated it if I could have updated my existent Sonos system to support thread. But as the landscape is right now, I had no other choice.
As most customers will probably have multiple Sonos speakers around the house, turning them into thread border routers would make sure my thread network will work immediately across the whole building. It would honestly boost the value of my Sonos system. I am not sure if an upgrade is technically possible, I have not checked all the details of thread. But if it is, I believe Sonos should definitely execute on this.
To the audio purists I would also like to say: yes, I understand Sonos is making “sound” only, however they are an entire part of my smart home as well. You can’t see one without the other.