I have no doubts that the F10 will age well and gracefully. All but the blunt front end I think will look modern, and then classic, for many years to come (or forever).
Regarding the W212, the jury is still out on timelessness. On one hand, in theory, you're completely right, eye-arresting designs, with strong details usually don't age well. However, I think M-B's "plan" for this car was to make it look revolutionary to the brand, but fuse in many classic M-B character elements (it reminds me of an evolution of the W126 inside and outside, in terms of character and charisma very much, my assumption that M-B did that intentionally).
Right now it's at the stage where people are starting to "get" the design, and are falling for it as they see it show off its dynamics in different personal elements. Of course, there are also those who don't or never will "get" it, or don't think there's anything to get, therefore they'll never be on board.
Whether it becomes a design that was too brand-revolutionary, trendy, and superfluous for its time time (a'la E60, although E60 was a trendsetter, to be fair), rendering it to be hugely popular initially, then an eye-sore and dated-design later, or something that masterfully uses its dynamics when necessary (to impress), but remains mostly an understated and sophisticated design as they become commonplace, allowing it to age gracefully and timelessly, but keep an enthusiastic presence and look eternally (this is in fact the task that designers of the legendary Benz and Bimmer Sedans are faced with.... Walking a fine line, which calls for a perfect design), we will see.
The thing that the W212 has that IMO is working for it, even with such a fussy design, is *CONFIDENT, STRAIGHT, CRISP, LEAN* lines.
The CLS, as gorgeous as it probably is (it's been eating away at me, to be honest), is something far more funky to the eyes, as the lines have cross personalities, with a wedge persona, and a character line that is reverse-wedge, and doesn't start at a solid point. This I think is what may make that an eye-sore after being viewed very much (very arresting initially, but not as conservative as an E-Class, hurting the CLS's longevity factor).
Then you have the new A-Class, and upcoming W222, with will have NON-Straight lines, and very fussy details, which can be disasterous to timelessness. IMO if there is a Benz that will be timeless that is coming out in this gen, it will be the W212, and the W204, at least that's what I think (until I see the others in person).
Me, being someone with some stake in it at the moment, I'm gonna be on the cheerleading sidelines, and assume and think that the W212's design is enough of a masterpiece that it will cross the two elements: Eye-arresting, and gracefully and conservatively, timelessly aging. Funny thing is, last generation the roles were reversed.... And IMO right now, if you put an E60 next to a W211, or an E65 next to a W220, the Benz Sedans look timeless, and elegant, while the Bimmers look like "yesterdays news" eye-sores.