Well, for one, you are assuming Brand B makes a camera "at least as good". Consumer sentiment would indicate otherwise. Brand B certainly has a better image sensor...but its camera has a variety of issues, say, with white balance, LCD screen rendition, AF system, buffer unload rate, etc. Brand B has great glass, but it is not as good as Brand A. Brand B's camera is phenomenal for some things, but Brand A's camera is phenomenal for just about everything, with a few caveats at really low ISO...
So...IS it really true that Brand B makes a camera "at least as good" as Brand A? Technologically speaking, they certainly have an edge. Overall, consumer sentiment seems to indicate Brand A still makes a better camera. And that sentiment has nothing to do with brainwashing or existing gear or anything like that (we've seen plenty of cases of switchers here on CR, where people have literally dumped their entire Canon kit and switched to Nikon or vice versa.)
As for price/performance...the D800 does have a phenomenal sensor. However that camera is clearly not as viable in as many use cases as the 5D III. Its gargantuan file sizes has turned more than a majority of wedding photographers off. It's lackluster frame rate without spending additional money on a battery grip (which normalizes the price gap and offers a size/weight ratio benefit to the competition). The poor full buffer clear rate of the D800 creates a lag in your ability to keep shooting, where as Canon cameras just keep on plugging away.
If you consider sensor the singular factor that affects a camera's competitiveness, and it actually turns out that sensor is indeed the primary thing that affects IQ for the kind of photography you do (I can think of one case where that is probably always true...landscape photography), and you are completely unwilling to wait and see what Canon does...then dumping your kit and jumping ships, or straddling both the Canon and Nikon ships, is probably the solution to your problem. Does that mean you are getting a better price/performance ratio? Well, if you do not yet currently own Nikon, and do own Canon, your price point for the D800 for better low ISO IQ (and ONLY better low ISO IQ) is a hell of a lot higher...you need at least one comparable lens. If you just pick up the competing Canon camera, even though the single-item price point is potentially higher (depends on whether you actually get that battery grip for the D800 or not), the total cost to upgrade and not jump ship puts you at a better price/performance ratio.
Rationalism isn't as cut and dry it might seem when one only factors image sensor into the basis of image quality and bang for the buck. I'd say the market is pretty rational already, and that photographers already are purchasing the camera with the best price/performance ratio for the kind of work they do. If the D800, D600, etc. were hands down far better cameras than the Canon alternatives, consumers would be buying Nikon.
Everything you say is of course true, but sadly everything you flag up as of limited appeal is of great interest to me and the Canon advantages I don't see as advantages. Thats why I am dissapointed that Canon have chosen to dump all their loyal IDs customers in favour of video and sports use. I am really not bothered if a camera of mine took just 1 shot per second, frame rates and buffer sizes, like high ISO noise, is of no interest.
I fully understand that something like the 1Dx will be perfect for some, all I want is for Canon to consider both sides which at the moment they are not doing.