Not a Sonos employee, and can only guess since Sonos has never commented on this before, but I would think there are several limitations to this. Firmware?  Could be.  It might take up a bit of the existing memory, and certainly would be development and testing costs.  Hardware?  Probably.  Sonos uses 5.0 GHz to sync the speakers, currently limited to 4 devices.  Not sure if the exiting hardware could handle a bump up to 8 total devices (or whatever would be needed).  Also not sure if wireless traffic would prevent stable connections.

Lastly, there is the market size constraint.  Not a lot of people are looking for a 9.1 type system anymore.  And those that do are often going to go with a wired solution. If you’re going to put that kind of effort into it, cutting into the walls and such tends not to be as much of a concern. 

I would not count on anything like this any time soon.  If they moved in that direction, they are likely going to start with a 5.1 setup where you can use Five’s for front L-R and perhaps center as well.  5 total speakers.  Then may 7.1 or 5.1.2 (atoms ceiling speakers)

I’d be happy if the split the front L-R out and allowed Fives! I’d be excited about that. Hope that’s a possibility and comes with less constraints. Thinking about what I was imagining, it was really being able to have independent L-R broken out from the Arc, so that something like rear Fives doesn’t sound disproportionate to the front soundstage.

So maybe I need to update my request of Sonos -- Please allow 5.1 where front L-R can be independent speakers from the Arc, and the Arc just runs center and Atmos. :)

I would guess that they come out with a new device to connect to your TV, possible with a center channel built in, rather than turn off half the speakers on the Arc or Beam to accommodate this.   Although that would allow for the atmos speakers in the Arc to be used. And to be clear, I have no idea if this is even on Sonos radar.

Hmmm. I kind of think that route would be a bummer, because the brand new Arc would be partially obsolete or useless to my set up if they made a stand alone center channel that support L-R but wouldn’t allow me to continue to use the Arc for that purpose. Anyways, we shall see!

Still would like to know where the current 5.1 limitation rests from Sonos.

True, but you do have the option of selling or repurposing your Arc.  Think if you’re a new customer, are you really going to want to buy an $800 soundbar with then intention of not using 6 of the level built in speakers, and spending another $1000 on Fives?

Yeah that’s why I thought 9.1 would be awesome, because the Arc could serve its full current purpose (FL-C-FR), but you could add in Fives for L-R Wide. But probably a pipe dream if you’re correct that it’s that many causes of a limitation.