A duty cycle is an amount of time the device can be in operation, in relation to a necessary period of rest before it can become capable of performing the cycle again. Most commonly that is related to heating of electrical components. There's nothing about the fan that needs downtime... it either survives or it doesn't. At work, even our most torturous vibration tests of critical manned spaceflight components are just a couple hours long. If it survives the couple hours, we're betting people's lives it will survive a very very long time in that environment (essentially indefinitely, barring some other effect on the material, such as UV degradation, thermal cycling, etc.). A fan can not have a duty cycle.
Also, the cooling system is designed to fail with the fan on. In hot environments when hauling, the fan runs basically all the time. So your instance of the fan running constantly doesn't really represent an unusual case that should have created an unexpected failure. Keep in mind there are 100,000 of these trucks out there, and millions of other vehicles with plastic fans of various ages. If you're going to stick with the "plastic fans suck" theory, the more difficult part is explaining why millions of other ones work just fine.
I am unconvinced it's driveshaft vibration when none of my bolts save one were loose
I know you're unconvinced, and ultimately it's playing odds so there is a small but present chance you are correct. You could have had failures of several "unrelated" components (fan, alternator bracket, water pump, u-joint) in a close amount of time, and they were each just individual components reaching their lifespan. "Truth is stranger than fiction", and all that, there are going to be edge-case stories like this about everything we could imagine. My point is that if they are all related to a single root cause driveline vibration, you have a high risk of fixing these symptoms, only to experience them again, or worse. When it happened to me, I think I just wanted to believe they were unrelated, even though I had thought logically they probably were connected.
You said you were likely to get the driveshafts balanced anyway, so I guess you'll know more once they tell you what condition they are in. If they are in good shape, you'll be vindicated. If they aren't, you'll have some new things to consider.
Which one bolt was loose? Bolt in what component?