I think when people like TDP start talking about the theoretical DLA, the practical effects in actual use can be misunderstood, and leaves people thinking that there is more of a problem than there really is. However.............So 90D with 32.5Mp crop sensor would have the same pixel density of a 83 mpixel (32.5 X (1.6)^2 = 32.5 * 2.56) FF sensor.
With this pixel density DLA according to TDP is f/5.2 A 90mpixel camera would have a DLA f/4.8 to f/5 tops. Which means it needs f/4 lenses or up to f/5.6 as the worst case. Maybe an 80mpixel would be a better choice?
Or this is the rumor of 500mm f/2.8 is all about
As the pixels get smaller and smaller diffraction does begin to bite, and I'm about to put up up a tread to justify my comments in this thread earlier about the 5DS at around f/16, which in itself is not a greatly used aperture, but as the same diffraction begins to creep in at more common-or-garden apertures, like f/8 for example, it is going to become as real issue where people are looking to achieve full DOF and the full IQ potential of their very high resolution FF cameras.
It's written all over the web how an ultra high mp DLA camera will still record more detail than a lesser one that is not diffraction limited, people - even - leave - a - space between their words to emphasis their point, and indeed this can be demonstrated by shooting a suitable test target three metres from the camera in good light and viewing the high mp cameras at it's full output size. However get away from studios and test targets and shoot in real "landscape" light and view the image at real, practical output sizes and the picture is not quite so rosy. ( Or more accurately, not so blue ).