I don't think it's processor tech that's plateaued as much as software load.
A lot of people out there are still happily using socket775 and AM2+, part of why the weak cpu's of tablets and smart phones work is that the every day uses of them don't need a high end cpu.
But I wouldn't worry about desktops disappearing any time soon, all those lovely "app's" people love are all created on desktops after all.