Your comparison between the tablet OS'es and DOS are not quite correct. DOS could only run one application at the time, Android and iOS can run several even if they run in fullscreen. I don't see Android coming in on the desktop side in the future but if it did then removing a built in limitation that only exists for battery saving wouldn't be a major alteration of the OS, it would just be another version. Multi-tasking have nothing to do with the ability to put two applications side by side on the screen.