You are right in that everything has a finite life, but......
Seals used on the crankshaft, valves stems, etc last quite a while under more extreme conditions, maybe 300,000 miles. So why can't water pump seals? Bearings used throughout the engines last, for the most part, 300,000 miles, why can't water pump bearings? In short, there isn't anything about water pumps that will cause them to fail that isn't also true of an engine.
Generally speaking, a simpler design means less to fail and longer life. A good example would be turbofan jet engines used on today's airplanes. A bunch of fans mounted on a shaft that will last somewhere around 25 times longer than the radial piston engines that they replaced. A water pump consists of a pulley attached to a shaft mounted in a bearing with an impeller mounted on it. You can't get much simpler than that.
If you want to quibble about "lifetime" then you could define it as the point where 50% of the properly maintained engines would require repairs that would cost more than replacing the engine. My guess would be somewhere around 250,000 to 300,000 miles.
What I"I'm saying is that there is no technical reason why automotive water pumps couldn't be built that were more dependable than the engines they're mounted on.
Jack