I was saying that the same focal length lenses are really not that much larger or heavier. My mom's 300mm f/4 lens for the 67 format, is 3.4 pounds, and its made of heavy brass. Yet its image circle allows an almost square sensor dimension that is 70mm wide! That's hardly cubing the weight of Canon's lighter and more modern designed 300mm f/4 EF lens. I really don't see this math adding up, to be honest, because you're forgetting that you wouldn't need to match the FOV...
I'm not sure why you say you wouldn't need to match FoV. A 300mm f/4 lens for medium format is not the same as a 300mm f/4 lens for 35mm. Those are two radically different lenses, by FoV. The entire point is to match FoV, that's why were constantly referring to APS-C crop factors and multiplying lens focal lengths by them...FoV is everything. Assuming a 55x44mm sensor, your crop factor is once again 1.63x compared to FF/35mm. So a 300mm lens for medium format is a 184mm lens in 35mm format.
Going in the inverse, if you are interested in an EQUIVALENT medium format lens to a 35mm 300mm f/4, then you actually need a 490mm f/4 lens for medium format. Now, assuming we use all the same technology that Canon has for their 35mm format lenses, were basically talking about the EF 500mm f/4, albeit with a larger back barrel to support the larger image circle. In this case, a 500mm f/4 lens for medium format is probably going to weigh 7.3-7.5 pounds, vs. the 2.6 pounds for a 300mm f/4. That, as it turns out, is a 2.85x weight difference.
But it doesn't stop there. You have to consider minimum focus distances. A 500mm f/4 lens on MFD is a SHORT telephoto lens, not a long telephoto lens. Minimum focus distance of a 300mm lens on 35mm format is around 55 inches. The minimum focus distance of Canon's 500mm f/4 II lens is 150 inches. You would need a greater optical power to allow a closer focusing distance to actually achieve total parity, which means a greater curvature in the lens elements, which is going to increase the material in each lens element. That will further increase weight.
It's doubtful that the weight of such a lens would literally reach 17.5 pounds (which would be the actual cube of 2.6lb), but it will certainly be much larger and heavier in order to achieve parity with the 300mm f/4 lens for 35mm format. You can't compare 300mm f/4 lenses in both formats...you have to account for the crop factor.
If a medium format sensor say 40 to 45mm wide, has 100 megapixels, then you really wouldn't need to be cubing the weight of a 600mm lens, to get similar magnification at the pixel level, to what you get with the 5D3 with a 600mm lens. If you don't need the full 100 MP, they could simply adopt Nikon's approach and allow you to shoot in crop mode. (That would be the common sense approach). Who cares if the actual focal length is shorter if the pixel size is similar to begin with?
Take a look at the average size and shape of modern medium format digital bodies. They are not only larger in width and height, but they are also considerably thicker, two to three times thicker depending on which sensor back you have installed. The weight of the body itself would be considerably greater than a 35mm format DSLR body. Ergonomically they are not as easy to hold.
And, again, you cannot compare a 600mm lens for 35mm format to a 600mm lens for MFD. Your completely ignoring the crop factor of the 35mm relative to the medium format. You would need ~1000mm lens for MFD to compare to a 600mm.
Additionally, by "just doing what Nikon did", by digitally cropping, you then just have a 35mm frame, so what's the point of having medium format in the first place? The entire point of using MFD is to get the larger FULL frame, not a higher density cropped frame. You want both larger pixels AND more pixels AND a larger sensor diagonal.
These are the reasons that pros, who already use medium format (it isn't something they "will" be using 10 years from now...they HAVE been using it, for decades), use it for studio, portraiture, landscapes, and architecture. These cameras ARE big and relatively heavy compared to 35mm or APS-C format cameras. Comparable lenses ARE larger and heavier, especially those that achieve similar IQ...it's a lot harder, requiring even more precise optics and manufacturing tolerances, to produce bigger lenses that achieve the same level of IQ as smaller lenses. The larger the optical elements, the more difficult it is to eliminate optical aberrations. That's WHY Canon's big white lenses are so expensive...they require much higher grade optical glass, and much tighter manufacturing tolerances, to produce the level of IQ they do. Imagine ALL of your MFD lenses costing that much...