Look at sales of Macs. Growing in general, though slowly. Then look at PCs, sales decreasing. Then consider that Macs probably have twice the useful life of PCs. Do you think user time on PCs compared to Macs is increasing? I don't think so.
Twice the useful life my rear, at least so far as doing anything remotely intensive. Considering most Macs are far behind PC in terms of performance at the time of purchase, professionals are going to have to replace their entire machines far more frequently than someone who buys a high end PC. The PC I built 4 years ago is still going strong, my mothers, brothers, and my own laptops are all still running smoothly after 2 years.
PCs don't break down nearly as fast as some people like to believe.
Sales growth doesn't necessarily mean PC is dying, maybe its because you can get a powerful PC for half the cost of a Mac and people aren't replacing them as often.
And it may be a small niche of the whole market, but I'm seeing more and more creative professionals abandoning Mac and getting a PC because of the same reasons. Cheaper, faster, just as reliable.