I think this is difficult to say. Let's use a practical example. When the Jaguar XJS was first released it was slated, from every angle. It wasn't a purists Jag,what were they thinking if that was the replacement for the E-Type. It had a baptism of fire. Now, XJS's are very much a collectors item and sell pretty well.
When I was a teenager, the E36 M3 was the iconic sports sedan/coupe of it's time,not a legend then,but sought after now (ok,the German Spec ones more so). I think whether a car is iconic or not will be up to the motoring purists, of which many of the people on this forum are. It's not about the race pedigree, the looks or whatever, it's what that car was perceived to be when it was in production.
I do think that there are probably too many M cars in general and BMW now stick an M badge on anything,and please don't get me wrong,I am not dissing other M cars,but M cars of now,it would seem,don't have the same mystique as M cars of the 80's. Why, because you had an M5 and an M3,and the even rarer M6. That's 3 models. Now you have a 1M, M3, M4,M5,M6,X5M,X6M,surprised there isn't an M7 yet. So the mystique is gone. Doesn't detract from the fact that there is something uniquely special about an M car. But is the E30 M3 as much of an icon as the original 328 from the 30's or 40's? No.
In a nutshell,cars become iconic for a whole different raft of reasons.