Even the most superficial examination of the digital marketplace is increasingly evident that the desktop OS is no longer the competitive playing field of opportunity, and that mobile OSs are where the puck is heading.
At the moment, no. Mobile OSes are an addition, not a replacement. They are designed to enable media consumption and comm on cheap hardware. But there are still users out there that want to use their computers for work.
To run CAD, CGI, serious programming, typesetting, image composition on iOS, you would not only need a port of the applications into completely new user interface metaphors, but you would also need to open up the hardware. In short, you would inherit all the bad things that constitute the hardships of desktop operating systems.
You may be right that in a few years from now, we will see simplified home computers with iOS like operating systems and a focus on web surfing, media consumption, light home office and gaming. Also, desktop operating systems will have to change (new file system metaphors, always on, no file saving any more, decentral backup strategies etc.). Still, I don't see a conversion any more than between washing machines and dish washers.