The thing is they don't have to replace
a computer with a phone or tablet just supplement it. Once that is done then people think well I mostly check email and facebook on my phone/tablet no need to get a new computer I think I will just hang on to the one I have for another 5 years... Phones and tablets are replaced in very short cycles <= two years. Computers I am sure are much longer than that at this point and it isn't even worth an internet search to confirm that is the case
All you have to do is look at the mobile app market; people are now performing lots of computing tasks on-the-go, on their mobile device. Thus, for many people, with so much activity now possible via their mobile device, what's the big rush to upgrade/replace their PC? This trend will continue as the mobile devices become more powerful and continue to do more.
But I don't understand what this big shock is all about and the nonsense that Windows 8 is to blame! Yes, back in the day when all your mobile phone did was make/receive calls and you were dialing into BBSes and other proprietary networks, of course new Windows versions drove the PC sales market because computing was on a PC, period. But that is no longer the case, so how can you expect Windows to drive PC sales when people are no longer as reliant on a PC as they were in the past?
The bottom line is that the computing platform is shifting and where one platform is shrinking, others are growing.