Saying all MB models in general become classic is a pure nonsense.
There are classics in BMW & Audi model line as well.

And there are many MB models that will never become classics: R-class, A-class, B-class, C SportCoupe / CLC, CLS, ML, SLK, etc.

Today cars are less classic icons than they were in the past ... Cars today are much more a consumption goods. Technology is advancing rapidly (especially in the premium segment) - so a 10 year old car feels much older then in the past. Hardly a classic - more like a likable 10-year old car. And mind production is also increased, so the cars (except models like AMG, M, RS, SLR, R8 etc) are much less exclusive.

IMO not only BMW are suffering from this problem but MB & Audi as well.


*****

Regarding the design / styling: I find all 3 brands design-led, and expressive design wise:

BMW - avant-garde Teutonic (postmodern / deconstructivism); the most pioneering & fresh ideas; breaking norms + often controversial; respected in designer world.

Audi - modern Teutonic (techno-design); classic aesthetic; step-by-step safe approach; academically perfect.

MB - neoretro Teutonic; classic MB design with modern elements; more daring approach than Audi but not close to BMW; good but not special design.


But the cards may change in a few years. Let's wait.