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BMW have been without supercar in their portfolio for 27 years! Since 1981 when M1 production ended (and it was in production from 1978 - 1981).
8-series & Z8 were not considered as supercars. They were just flagship GT & roadster. And their successors are coming in the near future.
So, in this 27 years without supercar BMW managed to gain the sportiest image within premium brands, and even became #1 premium brand in the world (saleswise), and managed to retain the sportiest image. I don't think absence / presence of supercar changes everything. Because BMW did not gain sporty image because of that few years when the had M1 in their portfolio!!!
Audi was not considered as sporty brand. Nor was Lexus. That's why they need supercars. It's an instant try to get sporty image. A supercar without sporty core models means nothing!
Btw, once more: MB has a super car, Ford has a super car, Nissan has a supercar, Honda & Toyota had supercars, etc. But that did not make these brand sporty-performance premium brands? No. It takes a lot more than that.
M models have a serious history and a long tradition - gaining an excellent image & reputation of hard-core sporty performance models: making a huge hallo effect on regular BMW models, which still are essential for BMW brand.
Right now there is absolutely no need for BMW supercar.
And like Scott said: supercars are 15-minute cars. Anybody still talking about SLR today? Or Ford GT? No. Or Zonda? No. Or FXX? No. And when LF-A will be launched, and some new Ferrari, Lambo etc will come out R8 won't be in spotlight anymore. Because new supercars will be better, faster, more exotic.
And with new design philosophy & other technological innovations BMW gained much greater (and longer lasting) publicity - and also having opportunity to present & communicate their sporty image - than non supercar could done that.
Current A8 & S8 was more damaging to BMW image than R8 would ever be - since outclassing E65 7er in home category: sporty luxury image.
Therefore for BMW's sporty performance luxury image new 7er and especially the new 8er are much more important & needed than a supercar. And I can say the same for M & CSL models - based on regular models.
Supercars by non-supercar brands are just wet dreams of motoring world ... In reality they are not as important as it seems.
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You can put lipstick on a pig, in the end, it's still a pig.

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