The VW diesels are still a cast iron block while the Mercedes have an aluminum block. I just don't see the longevity in a Mercedes diesel as in years past given the aluminum block.

That being said, the EPA has pretty much "killed" the simplicity and longevity of the diesel engine of the past. They have become extremely complex due to all that is required to meet emissions requirements. To keep these diesels going long term is going to require expensive maintenance/repairs.