does the air pump ever fail for reasons other than a leak ? And when I say fail I mean that the pump will still run but wont keep the car at height and yellow light comes on. And the year in question is an 06 with sport suspension.
Sure they can wear out, if you are driving an '04 the pump is 10 years old and should be expected to become weak and wear out over time, and as you suggest no other suspension leaks needed for it to become worn.
#1 failure of the WABCO ECAS pump is one of the magnets breaks free from the motor housing causin slow motof speed and drag. Overheat condition soon follows. You can disassemble and reglue (use JB Weld) the magnet and then add glue to the magnet that is still in place.
2nd common failure mode is a worn puston ring as this is an oil free compressor.