The W210 was a very good car, the so-called "Camry" of Mercedes-Benz in terms of reliability, except it had one huge problem: rust.
The true "million mile" Mercedes are probably the W123 diesels. Built very solid, they will still rust if you live in areas where road salt is used, but most cars will eventually.
It's weird how some cars are much more prone to rust than others. I had a Toyota truck in the 1980s and it rusted so fast you'd almost swear you could see it happening. Mechanically it never had any problems.
> "Camry" of Mercedes-Benz in terms of reliability, except it had one huge problem: rust.
Absolutely. The rust.. The rust..!!
> The true "million mile" Mercedes are probably the W123 diesels.
Yes, for sure. And the W124 diesels.
> It's weird how some cars are much more prone to rust than others
Different levels of anti-rust efforts. Where Mercedes-Benz truly angered their clients, was by coming up with a new model with a lot worse rust properties. (Well, they cut corners on other things as well, like the quality of the interior, but the rust would be the first thing you'd notice.)
MB had the know-how and the processes in place to make a car less susceptible to rust, and just decided to go with the cheaper option, clients and longevity be damned.