Lead sludge from blowby, also lead buildup on rings, combustion chambers, etc can gum up the works.

That's not to say all the old engines were used up at 100k, the IH sv series was known to go 300k between rebuilds. Those were carbd in the 60's. They were also industrial engines mmeant to chat away at 3 to4 thousand rpm all day everyday. High nickel blocks, forged cranks and generally heavy construction got them there.