You're just making my point exactly. You're getting 6 times the resolution out of 32 times the area. 35mm sensors are already common and that's already twice the area with better SNR. Maybe not all the 18 stops but close and it's also all semiconductor technology which improves for many other purposes than making sensors. There are also computational techniques to make the equivalent of a larger sensor from many small ones. We're coming at this from too many directions to not solve it. We'll soon need to have this discussion with 8x10.
The resolution of the sensor is pretty irrelevant. Camera and subject motion, the MTF of the lens, and diffraction are much bigger factors (once you get to a certain point). You're almost diffraction limited on modern APS-C cameras at f/8! The motion of your hand and body will cause a point of light to illuminate two sensor elements even at 1/8000 shutter speed.
Basically, DSLRs require perfect conditions in the field to actually achieve the sharpness stated by the camera manufacturer. A 4x5 sensor allows much more slop.
So you're doing 4x5 handheld? I don't understand your point. If there's anything digital clearly wins at is at getting you better results than film for a given physical constraint. It has better sensitivity so takes better advantage of the light, it has more density per unit of area so gets you more resolution per unit of camera volume or weight, etc. The only thing film still wins at is being able to cheaply build larger "sensors" since film doesn't get more expensive to manufacture larger. And as sensors have gotten better it's gotten harder and harder to actually realize a benefit from larger film up to a point where you need a large cumbersome camera in a very sturdy mount and really good technique and process to have a shot at it. I expect that within the next 5 years multi-sensor/multi-lens cameras and a bit of software to finish off the last few advantages there still are[1].
