Jack wrote:

There is no visual evidence -- to the naked eye -- of any
deterioration in old seats and springs that have allowed a faucet to
drip. The old seats and springs look exactly like the new replacement
seats and springs.

What is the typical reason that seats and springs fail over a period
of time? Is it usually the fault of the metallic spring, or the seat?
Thank You.

You'd probably have to investigate at the molecular level
into the rubber surface. The rubber hardens and doesn't
conform to the opposing surface or develops minute surface
failures. That kind of thing.

The springs aren't usually part of the failure.

Jim