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.