"The post PC era" is meant to refer to a world where (except in some specific professional contexts) people largely stop buying PCs because of a new class of mobile devices that takes over their workload entirely.
If I say "the mobile era" to people, that means something very different than "the post PC era". This is what language is for. We create new words so we can talk about new things, even when they're small changes.
Larry Ellison whines about the word "cloud" and insists it's just "servers". But "cloud" is about instantaneous provisioning and other infrastructure that lets you target an undifferentiated set of servers, rather than having to administer each server one by one. And unlike a "cluster" you don't generally own the whole thing. What word should we use, if not "cloud"?
Yes, it's still servers. Yes, the iPad is a mobile device. Yes servers and mobile devices are old. But whats your problem with using new words for the parts of these phenomena which are new?
Then I think we're a long way from seeing this era, simply because anything that requires more than a couple lines of text isn't just better on a keyboard than other input devices—it's vastly, insanely better (at the moment), a problem I don't see being solved in the nearish future. Rather, I suspect many if not most people will have one PC-esque device for keyboard heavy things and one or more "mobile" devices, using your definition.
This also assumes you're not counting laptops under your "mobile" rubric, which I don't think you are.
Anyway. I don't even see desktops going away all that soon because of one thing that gets lost on HN discussion: they're cheap: http://jseliger.com/2008/12/26/computer-post-desktop-or-lapt... .
Agreed, but we're just starting, maybe in a few years, who knows...
IMO, the term post-PC is a nice way to differentiate Android/iOS/WebOS/etc tablets from previous generation tablets that ran Windows or some Desktop version of Linux.
That doesn't necessarily mean that the laptops of the future will have a full-featured OS rather than a locked-down iOS-style OS. I mean, I'm sure yours will, and mine will, but will your mother's?
And maybe open/closed OSs will eventually converge. Look at Lion and Windows 8... I just hope they remain open!!!