Just taking a wild guess, I'd say engine longevity is 50% maintenance effort and 50% luck of getting a well-manufactured engine off the production line. With the Mercedes diesel of the vintage you're talking about, the "luck" factor is very favorable.
Keep changing the oil properly - that's the real life of the engine.
And it goes beyond pure economics to a few intangibles - one of the most important is personal safety.
Would you rather be in a highway accident in a 300D/SD, or a Kia?
1987 300D, 345k miles
1982 300D, 325k miles
Still going strong and smooth running......