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Two models have been considered: strict age cut (eg all machines older than 5 years are retired) and a do not resuscitate model ("DNR" - machines out of Maintech support left to die; up to that point they get repaired). The age cut presumably allows better planning of the physical layout of the data center, as the DNR model would leave holes by happenstance. On the other hand, the DNR model leaves useful hardware in place with minimal effort, but does assume that floor space and power are not factors in the cost.
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There are industry estimates for survival rates vs time. With a 5 year horizon, machines bought in 2013 only start to retire slowly in 2017.
Age cut:
Using a strict 5 year cut, here is the survival rate of the existing hardware (in 2019 it is all gone):
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