And those users are overwhelmingly the ones that tablets suit just fine.
That's the problem in this type of "PCs are fine, people just don't have to upgrade" argument. (And I've made it myself, before I really worked it through and took stock of just how far tablets have already come.)
The use cases satisfied by an old PC could all be satisfied just as well by today's tablets. Yes, the software support for keyboards, external displays and server-type services isn't quite there. But that's solely a software limitation. Not a limitation of the hardware platform. It will be addressed and more PC sales will disappear.
And the actual remaining PC-justifying use cases are precisely those where an old core2duo isn't "good enough" and simply buying some more RAM or even an SSD isn't going to obviate the need for new hardware for 5+ years.
So inasmuch as people still need PCs, they have to show up in the sales charts. And inasmuch as we don't, mobile devices are going to eat that market as the software evolves.
There's really no way around it.
Frankly, I think android and/or iOS are one good upgrade-cycle away from decimating laptop sales. If either or both really focus on the 'docked' use case -- better support for external displays and keyboard support to facilitate top-flight bulk text entry/editing -- laptops are dead to the general public.
If a student can so much as use a keyboard case and tablet to write a term paper or code up some homework as easily as on a laptop, it's over. And there's no hardware preventing that. It's software. If Microsoft wasn't so inept, they'd have been way out ahead on this.
And, frankly, between the potential of the docked use case and "BYOD" policies, mobile devices could seriously eat into the enterprise desktop market as well.
You build a device cradle, that connects to an external display, power and a keyboard/trackpad, and enables the OS and apps to automatically sense that connection and shift their interfaces accordingly and you'll see how few people truly need a PC anymore.