Time should not be a factor in the breaking of a jack - only usage, and it shoud not brake if used weekly if kept properly lubricated. I have a Ford T Model jack in my basement that still works great. I would call MBUSA if your dealer will not listen to you.
My 96 C280 has one blemish - right above the jack tube near the right rear wheel where obviously the jack fell.
The only jacks I have ever encountered that is worse than the Mercedes-Benz jack were the bumper jacks of the 50's and 60's - an accident waiting to happen.