I have had pretty good luck nursing along old water heaters. If I replaced one every time it got scale in the bottom I would need quick disconnects on the water lines and cord and plug connections. I usually pull out the bottom element and clean it with a shop vac about once a year.
My water heater is outside so a catastrophic failure is not really that bad. I am on a well so explosion is not very likely. It will just expand back into the pressure tank. I imagine that even in a worst case scenario, the PVC piping would let go before the tank.
