Thanks for many interesting responses. Two related ideas particularly appeal. Firstly:
2) The POTENTIAL for shallower depth of field, when print sizes larger than the limits of 35mm are used.
and relatedly, the idea that perceived DOF is related to the difference in sharpness between completely in-focus elements of the image and other parts: increasing the sharpness of the main in-focus elements increases the perception of other parts being OOF, especially if the greater sharpness invites closer viewing of the print.
In other words, the use of medium format rather than a smaller format can lead to some combination of larger prints, closer print viewing, or more careful print scrutiny, all making OOF effects more noticeable and so decreasing what I will call "perceived DOF".
Indeed, medium format is typically used for the sake of higher resolution, larger prints and prints that receive more careful viewing, so standard DOF calculations based on equal sized prints might be misleading. And standard DOF charts and CoC values are based on viewing rather small 5"x7" prints!.
So for larger, higher resolution formats, another approach to DOF comparisons could be this:
compare prints of equal l/mm resolution on the print, which in digital terms means roughly equal PPI printing, and then equal viewing distance since the equal print resolution allows that.
In that viewing comparison, the DOF formulas should be used with a CoC value related to pixel pitch, not directly to format. Current 35mm and MF digital offerings offer similar pixel pitches and so can perhaps be compared roughly with the same CoC, and the same is roughly true with film. If so, DOF scales rough proportional to aperture ratio and inversely with square of focal length.
So for example, 80mm f/2.8 in MF matches about 50mm f/1.1 in 35mm. Or to compare 24x36mm to 36x48mm digital, the focal length factor is about 1.4, so the equivalent f-stop for equal DOF changes by a factor of two: f/2.8 in Hassleblad-Imacon/Fujifilm's "48mm format" matches f/1.4 in 35mm format, and the brightest current AF MF lens, the Fujinon HC 110mm f/2.2 for the H system, matches f/1.1 in 35mm format.
That f/1.4 to f/1.1 equivalent DOF is probably shallow enough for most purposes.
P. S. As to the disappearance of those even larger f/2 and f/1.9 apertures, leaving only one current AF MF lens faster than f/2.8: the motivation for that extra aperture size might have been mostly more shutter speed rather than less DOF, and that is somewhat less of an issue with the higher ISO speeds of digital, even MF digital sensors with their lesser emphasis on high ISO speeds.