A blown head gasket will over-pressurize the cooling system faster than the radiator cap can vent it. That excess pressure will pop radiators, split header tanks, pop heater cores, AND blow out the water pump shaft seal. I've seen all those happen over the years. So the dealer sees a bad water pump and replaces it, unaware that its seal let go from the excess pressure. Head gaskets don't get better they get worse, really quickly. And then that problem becomes evident.