Sonos never announces things like this.  As to the probability?  It remains to be seen if it is even possible to turn off the R/L in a Sonos soundbar, or if the 5 GHz private network used for the surrounds/Sub can handle the extra bandwidth and still keep in sync with the video.  As of now, the slight buffer used (~30 ms) is on the edge of being noticeable as out of sync.

does not hurt to remind sonos of the many wanting this feature and ready to spend more money one additional sonos speakers to enhance their home theater setup ;)

Never said it hurts.  Though expressing a wish should not be the same as expecting it to be developed, especially when there may be technical reasons impeding its arrival.  Not saying you did this, but there have been plenty who did. 

I’d buy a three channel Amp today, Left-Right-Center. 

It would be a great solution for folks with spaces just a bit big for the Beam or Arc.

Set it up to use a 2 channel amp for height (like we can for surrounds now) or to use One SLs or better there.

Technically, that would still be a 5.1 system, but I I get your point.  (7.1 would be 4 rear surround channels along with the 3 front channels)

I would rather use Fives for the front left and right channels.  I don’t think I’d switch from Arc to 3 channel amp unless it also allowed you to use a 2 channel amp for height + amp (or Ones) for rear surround...but I still would have to consider it.

And I might even get an extra pair of Ones for side surround, although I don’t think my room is really built for that.

My bad ears make a solid center channel essential, went to a Beam instead of an Amp for just that reason. Replaced the Beam that was a bit less than needed in my space with an Arc only to find the center performing worse than my old Beam.

I was thinking about purchasing a pair of Sonos Fives to setup as a front left and right channel to go with my Playbar, Sub and two Play:1s. Based on this post I take it that is not possible so no reason to spend the extra money. 

That’s correct, you can not add front right and front left speakers to any Sonos soundbar, as they already contain those speakers/channels.