I suspect this situation will be down to what the R&D/ Marketing bods consider to be the most important feature for that time.
At one time, it was all about fps, AF and/ or megapixels. Now it seems to be about IQ, DR and ISO.
Without a doubt, the current 61pt AF seems to have satisfied most people - and most are happy with the 18-22mp range (note i said 'most' not 'all').
DR, IQ and ISO seem to be very much about the capabilities of the sensor combined with the light processing software used with the sensors.
The ability to improve the sensitivity of a sensor seems almost a given, working on how technology seems to improve. So, that would imply that the software needed to maximise the effectiveness of the newer sensor technology will be the guiding factor.
Will we see ISO listed at 408k or even 816k? I think we will - as H1 & H2 settings on the current 1D series model(s), because ultimately - for some strange reason - a whole load of people seem to want the ability to photograph a black cat in a coal cellar without a tripod or flash.
I like to push the ISO envelope as much as the next person (having spent most of my life doing shift work and using a camera at 3am frequently), but even in film days I found I could push a roll of 3200 to 12800 and hand hold at 1/60th second without too much effort. ISO 6400 on the 5D2 produced a much more acceptable image noise/ grain wise and with the IS on some of the lenses, allowed me to shoot as slow as 1/20th hand held.
So I'm more than happy with what is currently available - but I wouldn't complain if I were offered more