What is so different now than in the film days is there is no weight, size, susceptibility to wind, et al penalty for shooting with an 80 megapixel back over a 40. It seems to me, the conversation is a flashback to the old 4x5 vs. 8x10 arguments of yore except for the HUGE fact that, from 10 feet away, you can't tell if a photographer is using a 20 mp back or an 80. I'd be surprised if the 80 even weighed significantly more.
As for the cost difference, only the back costs more, everything else, camera(s), lenses (with a caveat here and there), tripod, cases, computer, printer, remain constant, so that the cost is not double to go from 40 to 80 mp, but more like 30 to 50% more. In the old days, that was not as bad as buying a Linhof Technika to replace your Wista Field.