Sorry to hear that you had a similar problem
All the people I have spoken to say that its just dumb luck many times , but if you service the tranny regularly it does minimise any future problems.
I have a w123 wagon 280,000kms no problems , My father in laws w116 350,000 no problems my mates 3 x w116 & 1 x 126 total 1,400,000 kms no problems all still original boxes!
So it may happen that they just may have been inferior metals gone thru the system and we were the poor saps that got them
If your box was properly o/hauled you will get many years or even never have issues with it again.:p
One thing for sure Merc do build good solid cars lets say 99% of the time , nobody can be 100% all the time.
gee your box cost $1900.00 thats $3800.00 in my money (Australian) thats at the high end , that is what the factory dealer said it could get to at the worst senario, Glad I found Sam Khoury.
Again sorry it happened to you just give it a service frequently say once a year or so and always check the fluid levels and it will be fine for quite a while.
Kind regards
BB
W123 wagon
W201 190E