In many situations, a multishot solution may be optimal. The sensor is moved between four exposures, so each pixel is exposed trough all four filters. That would contain all luminance information and also colour. But that would not be workable on subjects that change between exposures.
With this comment, Erik has persuaded me of the futility of using a monochrome back to produce standard color images by taking three images through three primary color filters, as with a color filter wheel system. Because whenever the camera and subject are motionless enough for that, the four exposure pixel-shift method can also be used with a regular Bayer CFA sensor, and then the same camera or back can also do regular color photography. Also, in the long run, multi-shot methods will work best in EVF cameras with purely electronic shuttering mode, working without either mirror or shutter movement and so minimizing vibration, to help keep alignment between shots.
The catch is that for now, Hasselblad only offers pixel-shift with a 50MP 44x33 sensor, and Phase One has not yet offered any pixel-shift option. My guess is that Hasselblad will take the next step by offering pixel-shift in a back or camera using the forthcoming Sony 100MP 44x33 sensor. Maybe in the X system.
The markets I see left for monochrome sensors are:
(1) Actual monochrome photography, for whatever artistic, technical, or poseur motivations.
(2) IR, UV, and other special filtering for technical usages, maybe including using more than three filters each with narrower spectral response, to gather more precise color information. (One use is forensic analysis of paintings, by analyzing the pigments used, either two aid restoration or detect forgeries.)
P. S. Plenty of self-employed blue-collar tradesmen need well over $50,000 worth of gear, between a truck and tools and such, and the annualized (depreciation) cost is probably less than the wages of a single low-level clerical employee, so I am skeptical of the line that "professional photographers cannot afford to buy this high end MF gear; it is only for rental houses and rich poseurs."