What are we comparing? Digital full frame to digital medium format is a ridiculously small step.
let’s start with APS-C and step up to full frame. The sensor size is multiplied by two (or 2.2), most sensors in APS-C and FF are sold somewhere around 24 MP, so there is no real advantage in resolution. There IS an advantage in depth of field for FF but it’s not as big as the advertising tries to tell us. In real life you miss a lot of shots at f/1.4 because the eyes might just not be perfectly in focus and the slightly larger DoF of an APS-C sensor could save the shot. I do NOT think I can tell if a portrait was shot with a FF or APS-C camera.
Now let’s step up from FF to digital medium format: the sensor area is about 1.5 times larger. Not 2 or 2.2 times. 1.5 times only. I told you that I can’t really see the difference between APS-C and FF, so it’s completely hopeless to identify any difference between FF and MF. They DO sell MF cameras with 100 MP (Fuji) and that is an incredible resolution, but I rarely publish anything north of 4MP (2560*1712) … why punish me and my laptop with those super huge files? Plus the separation / DoF of a 105mm 1.4 or 200 2.0 on FF is unequaled and doesn’t exist in digital MF.
They say that opportunity knocks only once but temptation leans on the doorbell ;o) and my temptation listens to the beautiful name of Hasselblad X1D (doesn’t it sound like music? ExOneDee?) but there is simply nothing it can do that a D850 with a 105 1.4 can’t do better - at a quarter of the price.
Things change radically when we look on the analog side of things. The “sensor size” of a Pentax 67 is about five times (in words: FIVE times) bigger than the 24*36 mm negative format of a classic 135 film. The detail and resolution of such a large negative or slide is just rivaled by the new 100 MP sensors now. And no full frame camera-and-lens-combination can come close to the incredible shallow DoF of the Super Takumar 105 2.4 in front of a Pentax 67 (another temptation).
If you want to shoot material for real huge prints, say two by three meters (seven by ten feet) , you might still go with analog medium format rather than digital. But in order to benefit from that great resolution, you need quite expensive lab gear, an expensive rotating (drum) scanner and again) a PC that can handle those huge files … the cost of camera plus lens is ridiculously low compared to that