Experts (both arm chair and otherwise) have long contended that one doesn’t really need high megapixel camera, unless you’re planning to print extremely large images and even then, there were ways to upscale image effectively. However in my view, this argument is no longer valid.
Because we now have 5k monitors, which have 4x pixel density than the current 1080p full HD monitors.
So if you have an 18 megapixel camera (5184px horizontal resolution), you will have to reduce full resolution image from your camera by just 64 pixels (horizontally), in order for it to be displayed full screen on a 5k display (5120px horizontal resolution).
Considering the fact that none of the cameras can produce clean images at 100% zoom, even at base ISO, you will likely end up in a situation where you can easily spot noise and other image quality issue on your 5k display.
Of course you can always do noise reduction on your images or up-size (in case you have camera with fewer megapixels), however results won’t likely be the same as they would have been, had your camera had more megapixels.
This is also true in case of video, which at the moment is only available in 1080p resolution in most DSLRs, even though many of the high end phones can shoot 4k video!
Even though 5k displays are extremely expensive to buy and run at the moment and hence quite rare, over the next couple of years, prices will likely fall and CPU as well as graphic technology will catch up as well.
While camera manufacturers are gearing up for the next megapixel race and are coming out with higher megapixel sensors and lenses which can actually resolve that sort of resolution, I get the feeling that the digital world is likely going to cause them more sleepless nights than the print days ever did. Because even though only a handful of images get printed large enough to show the true quality of camera and lens, 5k and other high resolution displays will end up displaying them at a much wider scale!