I'm trying to determine if this is something I would need/use and I'm struggling to give this an enthusiastic thumbs up
Full disclosure, I already have a 4 zone SONOS setup and I have several Chromecast dongles around the house. Each has their positives but they also have their negatives.
Don't get me wrong, I kind of like what you are proposing but...
1) I'm not sure I want another wifi repeater like tower/device lying around for which I have to hide the wires. If you could somehow make a unit that can draw power from a TV's USB port (like the chromecast) and is small/flat enough to tuck in behind the TV... that would peak my interest... That said, as a unit comparable to a SONOS Connect, I'd be ok with that, especially if it isn't white!
2) The one reason I don't own a Sonos PlayBar is due to lack of HDMI connectivity. Optical will limit you to 5.1 if ever you want to expand this offering (Axiom is, after all, a Home Theater leader
3) The best unit, in my opinion, in my SONOS system is the Connect:AMP. A 50wpc 2ch unit that allows me to power any pair of speakers and stream music. What would be awesome though, would be 1-ch 25/100/250watt units that can be paired in any configuration (2.1, 5.1, 5.2.2!). I'd compromise finding wall outlets for shorter speaker wire runs...
4) Related to the point above, if you incorporated the above units into your existing speaker line as an option
and for less than buying the two separately, that could work better for people with existing speakers not ready to trade in yet... I'm not sure I'd be willing to trade in my Totem's for a pair of M60's with Axiom Air integration just yet