Kirk, the implication that anything other than made with a 4x5 will look like crap up close is a bit silly.
I don't 'prevent' people from looking closely. But the simple fact is that most people don't. The simple fact is that the concept of 'normal viewing distance' has developed for a reason.
I ask about 'real' images because test charts at 300% or 400% aren't overly indicative of much. People in images move. Trees, leaves, grass, move. At 300% or 400% even a tiny amount of movement will likely be picked up. Will it be evident in a print? Even quite a large print? Call me crazy but I'm one of those photographers who still believe that the ultimate presentation of a photograph is a print, not something on a computer screen.
Blow a film image up far enough and you get nothing but a mess of grain up close, even at reasonable viewing distances in some cases. We didn't object to it then the way we do now. Medium format cameras have far bigger mirrors and much more significant mirror slap than a 35mm format DSLR.
Actually film grain, if rendered sharp, is very beautiful up close (low res digital not), even 35mm printed large. When I go to shows (my own and others). and I go to tons of them, I always see people sticking their nose up to prints AND stepping back to get the full view. It appears to me (and I have been at this for 40+ years with almost 100 of my own exhibits) that this notion of normal viewing distance has grown out of the early and very crappy early digital prints-a rational because the prints fell appart up close. I never heard of this concept before digital.