The market has pretty much plateaued. Pretty much everybody has a PC at home and work. Most households already have multiple computers. Heck, I know entirely non-technical powerwasher/gutter cleaner guys who have 2 or 3 computers. In fact, I don't know a single person older than 10 years old who doesn't have at least one Personal Computer of some kind.
Any commodity off-the-shelf PC will pretty much do whatever you ask of it (at least for most consumers). I used to replace my computer every year or two just so I could run modern software. I haven't felt compelled to do so for the last 6 years and even then I'm 50/50 on doing it. The rMBP my work issued to me is fantastic for virtualization, but unbelievable overkill for everything else I do (mostly email, word and web).
There's just not much of a reason to buy more machines outside of regular replacement rates due to failure and total obsolescence and new humans buying them as they get old enough.
It's not that PCs aren't coming back, it's that the constant growth in the market has plateaued.
Everybody was hoping China, India and Africa would explode 3/5s of the world's humanity moved into the middle-class and needed computers, but the growth has been far slower than was hoped and these first time computer buyers won't really be constantly upgrading like previous markets did -- the market characteristics are such that it won't be a simple repeat of the 80s, 90s and early 2000s.
Smartphones and Tablets are an entirely new segment and still growing (though showing some signs of flattening out as well). That's why they're exciting, because those markets are still building out and upgrading. But there are signs that those segments are flattening as well.
Tablets and phones are awesome, but they're definitely not a replacement for a general purpose PC. Even my mother and father, who're quite the luddites, regularly needs capabilities that don't work well on a tablet -- like doing taxes. Even if those things were magically fixed and working awesomely tomorrow, they'd still want a bigger screen than a tablet afford.
PCs aren't going anywhere, it's just that the market has to shift to sustaining the market not growing it (which is infinitely more expensive, meaning loads more money sloshing around in the secondary markets). This is fundamentally the problem that both Intel and Microsoft are dealing with. Apple escaped it largely because they created new segments to grow into.
Heck, the one new market segment that PC makers did manage to get into, netbooks, they managed to screw up so bad that the entire segment was dead within just a few years. (If you think of where netbooks needed to go as a segment, the Surface Pro would probably be a reasonable outcome, except that market is totally hosed now and Microsoft has to rebuild it).