Part of the.medium format look is the 56 millimeters on the long side of 6x6 or 6x4.5 formats. Size matters not just the number of megapixels but the physical dimensions of the format.
A digital image file does not have a "long side". What you say is certainly true in the film realm, because for any given print size you cannot change the output resolution
except by using a bigger negative. Larger negatives = better prints. But in digital you
can change the output resolution. When sensors were 6MP the point was academic, because 8 x 10 at 300 dpi is over 7MP, but 13 x 19 at 300 dpi is only just over 22MP, so at 50MP you can have just about any print size at just about any output resolution, and larger sensors no longer = better prints.
Another aspect of the medium format look is, as you say, lens focal length and entry pupil, and that is still there in the digital realm. But compared to the effect of larger negatives on print quality those things are minor. Plus, of course, in the case of the small-medium format cameras like the X1D and the Fuji the effect disappears, because the fastest Fuji lens is a 63mm f/2.8 and the fastest Hasselblad is a 90mm f/3.2.
