why not allow 2 one SLs or ceiling speakers as rear height channels for ARC?

Sure. Wouldn’t that be ‘a little software tweak’ to the Sonos Amp, which already has an HDMI ARC port?

Is there a bandwidth difference between a music stream, and a Dolby Digital or Atmos stream? Certainly the normal Sonos network is on a 2.4Ghz channel, and the bonded speakers are on a 5Ghz channel. 

Oh, and I do recognize that you didn’t use the term ‘a little software tweak’.

Eve if atmos uses 10 mbps that’s still nothing as compared to current WiFi capabilities 😀

I don’t know the real answer, so everything I say here is pure speculation. 

Sonos makes money out of selling hardware, plain and simple. If there was a way for them to easily create a situation that they could sell more hardware, don’t you think they would take it? 

I’d love it if your concept would come to pass. But I have to think there is some technical reason as to why they haven’t done something along these lines already, in their search for better profits. But I doubt that they would be likely to spill the beans here, where their competitors could gain insight on the way the company is thinking. It’s one of the reasons I think they don’t publish a roadmap for either hardware or software, trying to keep their competitive edge. 

And who knows, maybe technology is increasing fast enough that within X amount of time, they’ll release something like we want. I’m just not holding my breath. 

I see a lot of ideas, questions, and comments on why Sonos is not supporting various solutions with more speakers. Why not give us the possibility to add separated fronts and a center speaker? Why can’t I have 7.1 or 7.1.2? Can I add four in-ceilings for a more complete Atmos-experience?

I myself have thought of various ways to achieve some of these scenarios.

But I think the conclusion here is actually very simple. The Sonos lineup of standalone speakers is a simple one. It is the very reason that so many enjoy it: it is not complex but simple. A great solution for the more casual user. 
 

You hook up the speaker to the TV with one single cable and everything else to the tv. The Sonos soundbars all the way back to the Playbar just work with the TV as a build-in speaker really. Turns on with the tv, turns up and down the tv remote etc. 

If you start to care about how you can optimize Atmos with more speakers, or you actually believe you can tell the difference between Atmos over Dolby Plus rather than the full bandwidth over eArc - and believe it matters to you - Sonos already knows that their soundbars aren’t the right choice for you.
 

That’s the very reason why they have the Port in their lineup and why they made an effort to collaborate with manufacturers of highend receivers in the Works with Sonos program. To provide a possibility for tho enthuwho really care about these things to make an effort in putting together a high end system for their living room and then incorporate that in the Sonos ecosystem together with their more casual listening areas in the kitchen, bathroom etc. where Sonos’ standalone speakers will work perfectly. 
 

I have an alternative suggestion. Why not allow the customer to pair a second Arc and use it in lieu of the rears (One SLs e.g.)? Seems that would give you quite a few channels, solve the rear heights and make the surround channels more robust? 

Probably for the same reason they’ve never done a system with three separate speakers as front right, center, and left.  i.e. we don’t know, and I don’t think they’re willing to give away secrets two potential competitors.