I think you'll find that the parts of the hub that retain the free hub will have to be recreated on the outside of the drop and that's going to add more width to the chain line. The narrowing of the stays won't get much narrower than what we had on single speed bikes BITD.

Changing the wheel will take some additional engineering too. And when we get things like this to outside the norm, then manufacturing costs will be high because so few will buy one. Even if it proved to be overwhelmingly superior in several aspects. So manufacturer's willing to make it will change their minds when they don't see a large market share going that way.

I think there have been some bikes with a single stay. However those concept bikes were also too outside the norm for enough people desiring them to make it worth manufacturing them. Perhaps some still are out there in a niche market.