Yeah, of course, that's the main problem as far as I can see it, but by the sounds of it, that is becoming so negligible as to be meaningless, even for VR.
We would most likely get scaling chips fast enough to not matter, long before we have GPU's capable of natively rendering 20k@100hz.
I don't suppose Pimax would give us numbers regarding motion to photon latency for the 8k and 5k ?
I suspect, going forwards we really shouldn't bother with hard resolution at all, rendered resolution will scale as possible in game, and then upscaled to a panels native, and it's the pixels per inch that get interesting.
For regular flat screens it could be amusing to have a myriad of panels, as suggested by the Samsung LED wall screen.
If you want a bigger screen, plug in some more panels and you get X more screen size withouth sacrificing pixel density.