Not that I'm arguing, but what is the basis for saying the Mac has a five year lifespan and the PC has a three year? Is that based on some kind of accounting depreciation calculation? Or average time to failure? Or average time to obsolete?
The basis is statistical analysis done mostly by businesses and schools over the years, supported by dealer and sales information collected by Apple.
There are a variety of reasons for this, but the main one is that because Macs are both software-and-hardware designed by the same manufacturer, they meet the needs of those who buy them (and don't tend to upgrade them) for a much longer period of time.
This forum itself is FILLED with posts about G5s (around five years old), G4s (even older) and occasionally G3s and PPCs, and people still running Tiger, Panther and even older.
In the PC world, it's much more of a "sin" to fall behind the technology curve. For example, PCs tend to be much better than Macs for hard-core gaming, but that comes with a heavy ($$) price: your top-of-the-line video card today will be utter junk incapable of playing Super Ultra Destructo Waste of Time IV in a couple of years, forcing you to upgrade. Since most PC owners -- like most Mac owners -- tend to be non-nerds, they don't want to or know how to upgrade, so they tend to just buy a new computer.
Another factor is Microsoft itself. Every major iteration of Windows over the last decade (and the pace of Windows revision has picked up speed of late) has pretty much REQUIRED the latest or very nearly the latest hardware in order to operate "properly," again prompting a new round of hardware sales. Indeed, hardware manufacturers actually put PRESSURE on MS to speed up the releases of Windows, knowing it will help their business. Apple simply doesn't have that factor to contend with at all.
By comparison, Mac users with basic needs see very little reason to upgrade outside of a few free software updates. If all you want out of a computer is to be able to surf the net, check your mail, watch videos and listen to music (and that level of use etc), then your old Emac 1.25GHz running Panther that you bought six years ago can still do all that and more.
Bleeding-edge Mac users tend to run closer to the "three years" end of the spectrum, but we aren't really representative of "typical" Mac customers. For every one of us, there are hundreds of people still using AppleWorks on their "space bubble" iMac and perfectly happy till the day comes that they get ambitious enough to want to do something that machine can't do.