It's not just the cooling down, its all the resulting corrosion inside. They won't corrode as fast if you keep the moisture out of them. The ash , moisutre, air and metal don't play together.
If you keep it running it won't be unheard of to have it 50 years. I had one customer that had a losch that was 50 years old and I think stoker-man mentioned a EFM that was 50 years old.... You're buying a furnace for lifetime worth of use if you maintain it and don't move.
I'll give you an idea of the duratablity. I had one customer that lived in place called Shickshinny right next to the river. Their basement would get fully flooded about every 5 years or so. If the water was coming up he's take the conrtols and all the electric components off the furnace. Once the water recedded he'd get a hose, wash it out and start it up. That was Van Wert too. The coal he'd throw in mud and all. He did that numerous times... you can expect the same performance from a Keystoker I'd imagine.
Ours is a Van-Wet which they don't make anymore, its about 25 years old. My Grandmother has one that is 30 years+. It's auger fed, you don't have to hand feed it.