I think the issue is that digital had less depth of field because it behaves like a film that has a one molecule layer of emulsion, the thickness of film is such that the sharp area is slightly less sharp and goes out of focus more gradually as I understand it.
What I slightly take issue with is is the idea that full frame is somehow tougher on lens quality than smaller formats. Surely the current deciding factor is the level of enlargement from the sensor size. Historically speaking medium format and large format lenses have generally been less sharp than their 35mm equivalents they produce sharper pictures because of the smaller comparative enlargement factor for a large print. What we have now is full frame sensors that comparatively speaking have the same resolution as medium format but have to enlarged many more times to produce the same size print. The limiting factor is the lens. Surely APS sized sensors, despite the fact they use the sweet spot of a 35mm lens are put under even greater strain because the enlargement factor increases again. In the same way that high MP cameras are often a waste of time because the lens runs out of gas long before the sensor does.
I wonder how a mature technology like lens making will ultimately cope with this problem, my guess is as sensor technology advances MF sensors will be easier to make than 35mm lenses that can cope with the next generation of sensors.