PC usage isn't declining. There is just no reason to upgrade any more. A bottom end PC from 2 years ago doesn't give you anything more than a brand new one. Our dev workstations are 3 years old and still spot on performance-wise.
Everyone I know with a tablet has a PC and uses it heavily. The tablets are but a small diversion.
However, that is until you realise that Microsoft actually got this when they designed Windows 8 and surface. They worked out how to provide one device which fills both niches. Now Windows 8 and surface aren't perfect, but they will become in time...
Then the PC is dead. Oh no it's not - it's a tablet shaped PC. Again it evolves...