I suspect that many Mercedes owners are obessive/compulsive perfectionist types. I know that I am if I don't watch it.
What you are talking about is a concept called planned obsolence, where cars are designed to be obsoleted and worn out so that the company can sell new ones. I know that American manufacturers practice this. It seems to me that the quality of the engineering is what keeps cars viable over the long term, and that MB engineers their cars well, whereas GM and Ford do not.
I have decided (perhaps naively) that Mercedes does not practice planned obsolence, using as my evidence that (1) they sell high-mileage badges, and (2) they use their high-mileage cars in advertisements, as a tribute to their engineering, and (3) many, many of the older, high-mileage cars you see on the road and in the paper are MB (diesels). I think that they want their cars to last.
Now, you could say that all of these things are as a result of the fanatical owners going beyond the factory recs and we could debate that to a draw - neither of us having compelling evidence. We could both bring anecdotal evidence to the table, such as observations by a friend or something we read on the newsgroups.
It just seems to me that, given the dearth of direct evidence, we either rely on our own observations or those of others. Since I have no compelling observations on the matter, I must rely on others' recommendations. So it is a choice - who does one believe?
I belive the gearheads at MB. I think THEY are probably more obsessive/compulsive about the cars than even we are.
At least I am betting the motor on it.