irrc this is an external pump, so the most likely reason would be dirt ingress into the pump from the tank.
There is no filter in the tank, so 20 years worth of fuel with the crap that is found in the underground pumps will mount up, as well as rust particles, it only needs a minute amounty to damage the pump, and 20 years old will without a doubt have enough in there.
You can temporarily spin the pump in reverse by disconnecting leads and using a spare battery and wires etc, this will actually blow the crap back in the tank, you will hear this and the sound change which will confirm the situation, if you were having no running problems lack of power etc prior to each pump packing up, then it was unlikely to be because of the crushed pipe causing a restriction that the pumps packed up.
tabetha