I agree, that would be ideal (one usually needs different exposure times for the different narrow-band filters anyway). In addition to astrophotographers, I bet there would be a market among the more artistic photographers who wouldn't mind going back to b/w, and also surveillance etc where they need the best sensitivities. With the colour sensors we are essentially throwing away two thirds of the photons.
The conventional wisdom right now is that there is no market for B&W, and that colour images converted to B&W are 'good enough', though yes, there are quite a few people interested in using them for regular photography too. Every time it comes up in a forum there is clamoring for 'sell this at a sane price and I would buy one!'.
It has created a serious self fulfilling prophecy since the only B&W cameras on the market right now are expensive digital backs by MegaVision, AWT, and PhaseOne... MegaVision and AWT because they make scientific cameras and PhaseOne because they listened to people wanting an 'artistic B&W camera'.. though at 40k it makes a poor market tester.
Apparently Leica is planning on releasing a B&W version of the M10, which might give people SOME marketing data.. but their cameras tend to be overpriced/underfeatured status symbols so they make poor examples of what a larger community would actually buy.. not that much better then the digital backs and machine vision ones actually.
Of course there is always Maxmax's converted monochrome Canon cameras, but you are stuck with 500D/550D as your only options there... and of course no Canon repair shop will touch them.