It really depends on how the engine has been treat through its life, if its been thrashed from cold and serviced using poor oil or services skipped then it ain't going to do the miles,poorly modified doesn't help, the other thing is most of the engines that go early have been run low on oil at some point in there lives.
On the other hand a well serviced using good quality oils, treat with some mechanical sympathy when cold, never ran low on oil and ran on good quality fuel engine will last indefinitely.