Here is my $0.02:

Northern Europe, and Germany in particular, is a cold, generally wet place to live and drive. Lots of snow in the long, dark winters. High speed highways, and winding, narrow mountain roads, as well as tight Medieval City roads. All these features bred a number of special automobiles that we all seem to like because they handle well, brake well, have superior lighting systems, great turning radii, and superior stability at high speed.

High ambient temperatures are absent from the development equation. Stop and go traffic in high ambient temps are also not a part of the design's heritage. Neither was airconditioning until it became a "standard" option on Japanese cars, when MB, BMW, Porsche, Audi, VW and other well thought of European premium cost cars were forced to figure out how to make an effective HVAC system. I am not sure the present day system is really that successful yet.

So, I contest the statement that all those German engineers really designed the MB to run at 100 plus degrees C. They are much more focussed on making the cars work well at 100 plus mph.

To run cool at high speed you need a certain flow rate of coolant and air to remove heat from the radiator and the hot coolant needs a certain dwell time to get rid of its heat load. These features are juggled to try to get as small a radiator and fluid volume as is practical to reduce the frontal area (lower drag) and avoid carrying any extra inventory of coolant (weight). The air flow is a combination of the belt driven fan operation, and the high velocity of the air entering the radiator due to the car's speed. The coolant flow rate at speed is achieved with a pump of a particular size and fluid passages set to give fluid velocities that avoid damaging cavitation and erosion while remaining sensitive to weight concerns. These conditions typically set one or more of the design conditions for the system (high speed is not actually a typical high load point - high loads are all typically transient events unless you are racing).

Other operating points kind of have to suffer the consequences as long as they are deemed to be tolerable by the engineers. Unfortunately, all of this does not balance out to make the situation at idle or very low speed easier on the coolant system, especially when the ambient temperature is high (lowest air flow, lowest coolant flow, and lowest "delta T" between the coolant and the air).

To address the shortcomings of the system capacity at these "off design" conditions of high ambient temps and sitting in traffic (and having an a/c system installed), a bunch of Rube Goldberg junk is added. The things that are added are, an oversize belt driven fan with a complicated clutching system (electric or viscous, and either is a big dollar item to replace) to prevent the fan from eating too much power at high rpm (where it is not needed) or flinging itself apart. Then there are these electric fans with sensors and resistors and multiple speeds and so on, that will probably last for a few months if you run them continuously because they are not rated for continuous duty.

So, I believe the cars are intended to last longest and perform best at 80 degrees to 87 degrees C, and intermittent operation at higher temps is considered tolerable for the engine. It is definitely not a design feature to run there or the thermostat would be adjusted to run there all the time. Jim

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Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)