Small block parts are cheap because there is a huge aftermarket base to supply its popularity in the performance genre.

If MB's diesels had a similar large number of people rebuilding them, parts would be just as cheap with more non-OEM alternatives available. With no market to speak of, the cost of tooling to make parts is very slow to be recovered and as a result you have a $6000 rebuild instead of $600. Another good example of this is the California model's $55 air filter compared to the $12 federal version.

Part cost has nothing to do with longevity, its all to do with investment and profit.