Seals used onthe crankshaftt, valve stems, etc. are lubricated with oil, while water pump bearings are sealed without an external source of lubrication. Water is also erosive, which makes sealing and lubricating that much harder. If the coolant starts to pick up corrosion, the metal particles add to the erosion.
I like your definition, and come to think of it, automakers (at least Toyota, I assume all) have a definition of "lifetime" along those lines.
If those pumps were moving a lubricant instead of something erosive, then making water pumps more dependable would be much easier.