The alarms would start going off again when empty boxes reached the precision scales. Behold, an $8mil broken-fan detector.

Like someone said, two fans for live redundancy, little lengths of ribbon for visual failure indication.

I'd have thought a couple of fans checked once a day would provide less failures than are significant.

This doesn't seem like a reason not to propose the system, the alternate proposed solution would have similar failure modes too.

Two fans!