That doesn't automatically equate to PC sales declining like you seem to think though.
To be sure, we are no where near the death of the PC, but it is a well established, and undeniable fact
PC sales have declined significantly, with double digit percentages in recent years.
But PC makers still shipped about 70 million PCs in the first quarter of 2015. That's down 5.2 to 6.7% (depending on who's doing the analysis) from same quarter last year but still projects at least 275 million PC sales for 2015. Those are still HUGE numbers. And while the global sales figures may be down, the world's largest PC maker, Lenovo is seeing PC sales going up.
So it will be a long time before the PC is dead.
There are many who believe much of the global decline is due to the general unpopularity of Windows 8 so many PC owners are hanging on to their W7 systems. And many of those same analysts believe PC sales will turn around with W10.
I have no doubts the PC tower will get smaller as more and more move to µATX and smaller motherboards, SSDs, and more efficient CPUs and GPUs (thus requiring less cooling and smaller PSUs). But die? Not for a very long time.