Look at sport photography.....see all those people with the FF cameras and the 600mm lenses? With an APSC camera you can get the same field of view with a 400mm lens, but at the cost of image quality. A MF shooter would need a 1500mm lens to get the same field of view, but would have superior image quality. Obviously, this is not practical and MF will never fit this segment of the market.
Regarding the field of view, you are spot-on. But readers should bear in mind, a 600mm lens is still 600mm regardless of platform. With MF, you get the FF field of view, plus a great deal more, so that 600mm appears to be a shorter focal length. (You may have been making this point.)
As for practicality, I shoot the RZ. It is HIGHLY impractical for most stuff you might use a FF to shoot. It's heavy and awkward. It's made for slow, methodical tripod work. That said, I absolutely love it.
My longest lens is currently a 210mm, which offers the field of view of 102mm on FF. The resolving power of the lenses is pretty incredible, and with a 60x70mm negative, my desktop scans are north of 65MP. Pretty cool!