I would think something along those lines, as a "fitting" bike that can actually be ridden, would be highly useful in finding the "right" sizing for a given person. It'd add the dimension of riding dynamics (weight shift, hills vs faster spinning, etc). If they can get the weight of such a thing down to be tolerably close to a "normal" bike's weight, it might be a great tool.

If all the geometry angles, lengths, heights, reach can be adjusted for a dynamic fit ...

But then, I'm no engineer. Perhaps even 10-15lbs over a "normal" bike would destroy any handling dynamics improvement over a typical stationary "fit" bike.

I suspect that sort of thing is where we'll be, at some point. Light enough to be a reasonable facsimile of a proper bike, but fully adjustable in all the geometry dimensions to get it right for a given person.