Thanks, Kajtek. Sounds like at 52,000 miles, I shouldn't be in a hurry for a trans flush.
As an investigative reporter one would think you'd want a little better sample before reaching a conclusion. You might want to do a more thorough search on the site. Musikmann has a post in this forum about lab results of used fluid, and most members agree that fluid should be changed in the 5-speed someplace in the 50,000 to 65,000 mile range. Kajtek1 seems to have unnaturally good luck with his vehicles and I think he might agree that his experiences have been better than many if not most on the site, and not with just transmissions.
The "lifetime" in question is most definitely NOT that of the car, but of the transmission. As yet there is no magical fluid that is immune from friction material breakdown, metal shavings from use, degredation over time and possible moisture absorption (just like brake fluid). While there are those who have reached higher mileage, this and other MB sites are rife with tales of members whose "sealed" transmissions were done in the range of 100K to 125K miles. Many think it was just marketing hype because MB knew they'd go past the warranty period and probably over 100K even if they were abused a little.
Starting I think with the 07 MY MB started specifying a "one time" fluid change at 39,000 miles...on the same transmission. What reasonable conclusion might you draw from that?
There is a newer fluid for the 7-speed transmissions that is backwards compatible and about half the price, so it is possible your dealer intends to do a proper flush for $300 with that fluid.
That said, the great thing about owning a car is that you get to decide how to maintain it. I look for longevity and understand basic principles of mechanics, so my fluids get changed, marketing hype or no.
Bottom line, the odds are that routine maintenance of your car, including the AT, will extend the life of major components and hence, the car itself.
Take care and enjoy the ride,
P.S.: If you want something interesting to investigate, find out why so many late model MBs suffer cat failure just outside of warranty and why MB has extended the warranty -- but only for AMG models. The rest of us are left to twist in the wind and pay huge sums of money to replace something that really should last the life of the car...and usually does on other makes.