1. You might have noticed that MB, BMW, and Audi have all expanded their product lines into lower range cars. If you don't believe me look at the MB CLA class, the BMW 2-series, and the Audi A3.
2. You are caught up in branding and image but those things are not important to everybody. The Japanese did not re-badge Lexus, Infiniti, and Acura in Japan until recently and even then not entirely. Japanese don't mind paying top dollar for a Honda (that's called an Acura in the US) because they care about the product, not the brand.
3. It's true that Acura is derived from Honda, Infiniti from Nissan, and Lexus from Toyota. And it's also true that there are design similarities within these families of cars and even a few common parts from the parts bin. But you typically get what you pay for and you are getting, generally, a much nicer car with the luxury brand. And here's the real difference: the customer experience. Believe me when I tell you the customer experience is night and day between Nissan and Infiniti starting with the purchase all the way through service. These dealers all know their customers for better or worse