Obsolesence is a state of mind...usually fostered by marketing and sales people. And I really don't know what there is to "wear out" in a desktop machine in a few years. I don't doubt that they've been fully depreciated in accordance with an IRS obsolesence schedule, but that doesn't mean they are truely obsolete or "worn out".
umm greg, were you thinking the 130+ million PCs a year that have been bought the past few years were all for "new uses"? we haven't been on a rolling upgrade "cycle" for several years now?
I see computer life expectancy of about 5 yrs based upon expected component failure rate. Back in the real world, PC replacement rate since Y2K has been very sluggish @ about 1.5% per month or 7 yr lifecycle. So I see a catch up period between how long PC's are actually going to last and end user expectations of how long they should last. US businesses are coming to a rude awakening as all y2K compliant computers hit that hard wall of mechanical parts failures such as fans in power supply and CPU and bearings in hard drives really start to come into play.
Up until recently we were experiencing fairly fault free maintenance of fairly new computers. Right now there is not much rational to hang on to old systems based upon increased maintenance costs and accounting standpoint.