Basic Business Practices I suppose. Ideally you should be replacing things before they fail as Down Time costs you lots of money.
Also at the 5 year period in deployment you are at the end of the Tax Cycle where that hardware starts to cost you more money that you can depreciate on your Tax's so it's a Double Whammy where if it breaks you are in for a lot of Down Time and there is no Tax Benefit in continuing to use these devices that will have been replaced with new faster equipment that will allow your users to do more in less time.
So while they may be working perfectly now newer equipment is faster allowing more to be achieved in the same time allowing the company to potentially make more money or keep existing customers happier. And the Tax Breaks will be expiring very soon so the costs of using this equipment is about to go through the roof.
From a Purely Economic Side it's cheaper to replace every 4 to 5 years and retain stable platforms than to have things break which prevent work being carried out and require Overtime to be worked to catch up once the broken bit has been fixed/Replaced.
Just remember that if one of those Xeon CPU's was to fail now you would have to hunt around the Second Hand Places to find a replacement. Is up to 2 weeks downtime worth this?
The above is just a generalization as I don't know your company but they are very pertinent to every business no matter what it is that they do.
Col