How does how we display or print the images figure into this? There is a 24x60" panorama lying atop my Stylus Pro 7900 right now - a single shot from an A7r, printed on Epson Exhibition Fiber. That's a center crop (not even - most of the crop was off the top, so it wasn't truly using the sweet spot of the lens) from a 40x60" image! It's a high-detail landscape, and it looks as good as a very well-made 11x14" print from most 35mm films. A 200 lb 7900 is about as big a printer as most people are going to tolerate sharing their house with (actually, most people won't tolerate anything bigger than a 3880) - a 9900 (the next size up) is the size of a piano! The very best FF cameras can fully utilize even a 44" 9900 or iPF 8400, which is what it would have taken to print this image full frame.
In film terms, this sort of print was solidly in large-format territory. A 40x60" enlargement from 6x9 cm film at the top end of medium format was equivalent to printing 16x24" from 35mm. Yes, it was possible with the right film and the right subject, but for a high-detail landscape, you REALLY wanted to have a 4x5 negative (or, for a panoramic image, 6x17 cm).
We don't really need more pixels if we're already printing 44"! No screen is close to using what we have - even the lab-curiosity 8k displays are only around 33 mp, and 4k displays that are more available are only 8 mp. Any recent Micro43 camera can exceed not only the resolution, but the color and dynamic range capabilities of the best 4K TV. In my part of the country, it is nearly impossible to sell a print with a short dimension exceeding 24", because of wall space restrictions (occasionally, a gallery wrap can go bigger). The one exception is institutions - our local hospital displays some VERY large prints, but there aren't that many hospitals and such around. Places where architecture tends towards bigger walls than New England may increase opportunities in the 30x45" and 40x60" range, but such a print will nearly always be viewed from a distance, and is well within the capabilities of current cameras. No, I wouldn't push the A7r (or, presumably the D810) as far as 60x90" for close inspection, but I'd probably try it on canvas for an installation with a reasonable viewing distance, and locations that can hold a 60x90" print (or even a 36x90" crop panorama) are very rare indeed.
There is certainly more to an image than resolution, but the best modern digital cameras spoil us there, too. We have enough dynamic range that there is REAL detail in Zone 0 and Zone X, and Zone -I (?) and Zone XI both hold the hints of tonality that film photographers worked to find in Zones I and IX. Color has improved tremendously, to the point where no film ever gave us what we have today.
Although serious photographers have the best equipment we've ever had (Ansel Adams would have killed for an A7r and a Cambo Actus to give him movements, although his back surgeon would have cried), most pictures today are taken with cameras that are actually historically BAD. Cell phones' 4-5 stops of dynamic range, diffraction-limited resolution and terrible lenses are worse than almost any film camera ever sold. The most sophisticated iPhone is somewhere in the quality range of 110 Instamatic and Kodak Disc film formats, due to the relentless drive to make phones ever thinner - we will never beat the laws of physics!