i simply cant see the need for such high resolutions for any purposes except military or scientific.
we are not playing formel 1, but many companies and photographers think as it would be that way.
Reasons for 100MP Canon and Nikon
1. Because they can
2. Clients are impressed = bigger is always better
When will it end:
They will probably reach a limit to optics at a reasonable price and will stop at that point, or video will replace still cameras for advertising work, where 4K capture , (10MP), seems to be the future newest standard for anything but high end special effects work.
Ray Kurzweil is one of my favorite writers and has taught me to think, no only out of, but way past the box.