Sounds like your MB tech is part philosopher, which is usually a good thing. Tends to offer sage advice instead of strictly running by the book, I'll bet.
I agree with your comments about profitability and the intracorporate players all having a say in the end product. I also agree that external individuals have a difficult time assessing the amount of contributions each party makes.
I see your point about comparative quality, but disagree with your conclusion. I believe that an external standard of perfection does exist - customer expectations relative to price. It seems to me that MB is not merely reactionary, trying to beat the competetion, but proactive persuing the production of perfect products while pricing competetively. Pithy, I know, and probably painful.
However, this is speculation on my part, since I am not privy to internal workings of that company.
Perhaps the factory recommendations are substandard for excellent longevity, perhaps not. No one I have talked to has the numbers to prove it either way. It would be difficult to substantiate whether MB recommend a certain OC interval so that their cars will wear out at a certain mileage. Since we are external to the corporation, we cannot know whether or not they practice this somewhat predatory behavior.
I think you are right, it is a personal decision. When I assess the lack of hard scientific evidence in this area, I decide to go with what I consider to be the foremost experts on this subject - the designers and builders. It gives me a good comfort level. For others, it may be different. After all, as Mark Twain once said, a difference of opionion is what makes a horse race.
Now, if someone had some scientific analysis showing what REALLY happens during differing OC intervals, that would be the clincher, and all argument to the contrary would grind to a halt. Until that day, let the debate continue.