l understand your point but the engine is only one component. With the kind of high mileage you are referring to there are likely to be other issues. Figure in at least one transmission rebuild, several batteries, shocks, brake jobs and incidentals like alternator, ac compressor and starter and the so called "Nickle / dimeing to death" syndrome.Well, back to the subject, In my case, and others Im sure, we high mileage drivers may find 2-300K on the clock within six years. I doubt these trucks, frame rust and all, are doomed that early. What is more economical? an engine rebuild, or a new long block?
While it will be less expensive than buying a new truck, especially if you do the work yourself, you also have to factor in down time and the proverbial "I'm sick of this truck" feeling that typically kicks in after you've spent all that time and money fixing things.
If I were looking for longevity in a truck, l would move up to a 2500 Duramax. All the components are so much heavier and designed for long service.
BTW: When l was moderating DieselPlace.com, we had a member who had well over 500k on his Duramax, most of it towing, before having to rebuild his engine.