Most tablets on the market are nothing more but a digital newspaper device oriented towards media consumption. PC is a device that's productivity oriented. Just because most of the people can get by with tablets doesn't mean desktop PC will be completely redundant. Nothing can replace a mouse, a keyboard and a big monitor when it comes down to productivity and that's what's going to keep desktop form factor live and kicking for a very long time.
The only tablet that's worth being called a PC is Surface Pro, which, funnily enough sports x86 CPU and desktop OS. Now it's up to Intel to keep pushing power consumption down and IGP performance up for it to rival most desktop features. Full blown desktop PC will always do everything better (faster and with greater comfort, i.e. bigger screen, without clumsy input devices, etc.) for less money.
It's a natural evolution in PC industry. OEMs have to diversify their product lineup for the same reason why you don't see every car company produce only SUVs.
That's why Windows 8 is what it is, and it was a move in the right direction whether you like it or not.
Would you rather have a 10-core Ivy Bridge Extreme desktop workstation, a Surface Pro and Lumia 920, all with Windows 8 (assuming you're going to use all those devices) or the same workstation, and iPad and Android phone? Think people, that's what kept Apple going for so long, but now they're outgunned heavily by MS in that they've got more powerful/versatile devices and sell them for less money. Windows based custom built workstation will sport latest hardware unlike Mac Pro, Surface Pro costs roughly the same as 11" MBA, provides equal performance and doubles as iPad when needed, and having a phone that's natively a part of the ecosystem is just a plus, again for the people who can utilize it all.