We should know automotive business is very complex one. From idea, business case, giving project a green light, design, development, production, marketing, sales, after sales services etc. All this are phases which are very complex, need a lot of know-how and - a lot of time.
Today all these phases cost a lot. So a company has to make a lot of revenues to have enough money for R&D. And therefore the company has to expand their business / model line. Also they should be aware of future trends, and bring new interesting product to customers - otherwise they become ... Ford.
All models you have listed are a success for the companies. Neither of them failed.
Look - even Porsche is expanding: Cayenne SUV, Panamera "4-door coupe", etc ...
Yes, to quick expansion brings problems - usually in fall of quality. Even Toyota has experienced that recently.
But, Grim, I get your point. Sometimes such expansion of model line can hurt the brand image - especially when expanding down-market or in segments not compatible with brand image.
But that was not the case with German manufacturers / brand: non of them experienced damage to brand image. More due to worse product quality then to model line portfolio. Mind that MB image was hurt due quality problems - especially in all classes, not so much due introduction of A-class.
Regarding 9-series: scott just said it was an idea - it would be decided about in few months. We do not even now whether it will get a green line. They are just considering it.
BMW Group have always ambitions in sub-RR segment: therefore their plan was to acquire Bentley as well in late 90's. Now seeing Bentley being very success full, and Aston as well. They are revived the plans to enter this segment. Either with some upscaled BMW models above 7er (8er, 9er, Z8 etc), or now when a chance occured - with acquiring Aston Martin.
Mind that this segment is very profitable - profit margins are very high, and even sales volumes are counted in few thousends.
Mind VAG has Bentley & Lamorghini in this segment (Buggati is more a RR competitor), MB has a CL-class & SLR, and Maybach - maybe even with upcoming baby model. While BMW Group have no models in this segment - they want to keep RR highly exclusive, while BMW or perhaps Aston Martin to fill the gap between 7er & RR models (Phantom line).
It will be stupid not to enter this segment. With BMW or Aston Martin brand ...