Collateral damage when replacing a sink faucet cartridge

Our kitchen sink has been dripping so we bought a new cartridge a few weeks ago to put in it. We only just did the replacement this weekend but at least it eventually got done. When Andy turned the hot water valve on under the sink after we were done, we had no hot water. The cold was fine and all of the water was fine elsewhere in the house so we determined that the valve wasn't working. When Andy replaced that yesterday (with no sweat connections!) all was well until we looked in the old valve and saw the plastic pieces inside that are inevitably from our hot water heater, which we've been meaning to replace for a few months now. Even if the heater went out entirely, I wonder how long we'd wait to get a new one. We have become even bigger procrastinators than we used to be, although I never thought that was possible.