I agree that it might be time.
Let's not forget that when ANY car manufacturer states that such and such maintenance should be done within so many miles, they ASSUME that the average car gets driven about 12-15,000 miles per year. Thus when MB says change your coolant every two years, they anticipated that would be 30,000 miles max. But if you get over 40,000 miles per year, it might be safe to assume that the coolant has been exposed to heat and circulation for the same amount of time as those two years would have taken.
Besides, changing your own coolant costs only the price of the coolant - cheap insurance in my concept of calculated risks. And yes, I have done the calculations!!!!......Andras