If you mean exclusively on small factor touchscreens, sure, I agree. An iPad isn't going to replace a dedicated home cinema room any time soon, or a hardcore gamer's custom rig, or a CAD workstation at the office.
But for routine use, that ship already sailed. Smartphones are ubiquitous when people are out. Tablets are becoming ubiquitous around the house, for the kind of household that used to have multiple PCs or laptops instead. Bazillions of people are quite happy sending e-mails, checking Facebook, or catching up on a missed TV show on these devices, and for many of those people that already meets the majority of their needs.
Not everyone cares about playing AAA games on a PC (they have consoles for that) or running business applications (they go to work for that) or writing software. And to a first approximation, no-one cares about command lines. Real PCs aren't going anywhere for those who do want to do these things, but there's no point pretending that a water-cooled 4th generation i7 is necessary for reading e-mail.
Here's a problem. I can certainly look at email, facebook, or even slashdot on my iDevice. But at some point, I'm going to want to tell someone they're an idiot, and that is very clunky on the tablet/phone. Even when I'm mostly reading my favorite forum (s), I prefer my laptop so I can rant at someone without switching devices. And, unlike the commandline, that's something a normal person would be interested in.