Maybe because Benz actually designed a Diesel engine rather than try to modify a 350 gasoline engine

I think that we make good adequate products might be a better way to say it. Remember when people buy a new Mercedes they make sure it is maintained and that helps a lot towards it lasting a long time. Even a used Mercedes owner when seeing a coolant leak he/she is going to loose sleep over the chance of a hefty repair bill and will get it fixed ASAP. USA made car owner might just keep adding coolant until the leak looks like or dose cause a serious problem.
As Americans we tend to be abusive towards many things that we have and do not maintain them. What is the point of buying something top of the line if you are never going to change the oil,filters and keep an eye on the fluid levels.

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84 300D, 82 Volvo 244Gl Diesel

I agree. I worked on some of them back in the late 70s and early 80s. Blown head gaskets, broken connecting rods, sometimes broken crankshafts. I have had the threads strip out of the bolck on several blown head gasket jobs; and yes I followed the torque proceedure and used a torque wrench.
The fuel injection system did not hold up well either.
Another problem was that the dealership mechanics were not trained to fix them (one of the reasons they brought them to our shop).
I always wondered why the did not copy a proven design.

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84 300D, 82 Volvo 244Gl Diesel

I would too if they actually were modified 350 gas engines. In reality, they were just cr@ppy diesel engines.

The whole "modified 350" thing was started by people who know nothing about diesel engines.