There are a whole bunch of variables plus a little bit of luck involved in engine life.

If oilt is changed hot and changed often, coolant is changed every few years and things are inspected on a regular basis, the engine isn't operated in an abusive manner and you get lucky, then the sky is the liimit.

With todays engine machining and material improvement methods, todays lubricants added to the fact that all engines have fuel injection these days, a half million miles is well within reach for a properly cared for modern engine.