Fine… I’ll use your comparison as the example.
The Corvette is Chevrolet's Halo. It's soooo much of a halo, it's actually a brand unto itself that Chevrolet cherishes deeply.
Rolls-Royce has been the Auto Industry halo for decades. THE FINISH LINE in life's journey so to speak. Bentley served as a more obtainable sister brand, and protected RR from having to go "down market"... but that is no longer the case.
In circumstances like these, the Halo should not move down market to fill the gap between itself and what's beneath it.
Scenario:
Chevrolet places a V6 into the Corvette so a $30k-35k "value" priced Corvette could serve as the bridge between the rest of the Chevrolet line-up and the legendary Corvette.
Heresy? Yes. It would ruin/tarnish everything that "Corvette" stands for. It would be more intelligent for Chevrolet to build another Chevrolet vehicle (Camaro) and push upward to help fill this gap instead of diluting the Corvette image.
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By Rolls-Royce moving down market, it dilutes the brand because it changes/destroys (they would prefer I use the word "expands" but no) what the brand stands for = Absolutely No Compromises.
The less expensive RR would have some compromises or there wouldn't be a reason for someone to eventually buy the Phantom. If Rolls-Royce makes them both equal, then the Phantom becomes "over-priced" because you can get everything for over $100k cheaper.
The Cheaper RR models would also be used in comparison tests against SC’d Maseratis, Mercedes V12 S-Classes, Aston Martins, etc etc... which are all quickly approaching the $150k-$200k markets because everyone wants a piece of that growing market. These are far less rarified brands that actually have a chance of WINNING because RR doesn’t have experience in this market and the others already have a strong & loyal clientele aspiring upwards. RR does not have an aspiring clientele moving downwards. Can you imagine a Rolls-Royce getting beat by a Mercedes or Aston Martin when this is said and done? That would be THE END of Rolls-Royce's legendary reputation. It would become just BMW's Ultimate Luxury brand instead of continuing as the Industry's Halo brand.
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So, it’s just in my opinion that it would be better for BMW to be in those competitions with a 9-Series sedan or 8-Series coupe serving as the bridge between BMW & Rolls-Royce. If sales don't meet expectations, BMW can easily absorb the image bruise and make that vehicle into a limited run. Rolls-Royce is too high profile to pull that off because it would leave it with what it already had… the slow selling Phantom.