This is the video that made me ask the question.He answers at the 7 minute mark
That is why they need to make new ones of them again.Not that long ago California Dept of Forestry was running Cat equipment so old it had oval tracks. Also had full glass and air conditioning. But I think that is finally ancient history now.
Last time I checked, California equipment counted hours run per year so if it is a seldom used machine, you could hang onto it for a long time. And fire use did not count against those hours. Probably because the pollution saved by stopping the fire outweighs the small amount made by the Cat. Nevertheless those old dinosaurs are dying out one by one.