> "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.