Steel rusts, cars rust, fact of life, while cars rust a lot less than they did, many are scrapped due to being rusty beyond economic repair, MBs included.
Cars aren't made to last 10 years, that's about their lifespan.
I would say that MBs are made to last 6 years.
Most cars are brought on lease or credit over 3 years, once the lease is up the clean ones go on to be retailed as approved used cars, with a years warrenty or possibly 2 years.
After that, who cares?
The manufacturer has made their money, and anyone who buys a new one won't really care if it lasts forever as they'll be getting a new one every three years anyway...