All this "I would never" stuff is just plain silly or at least naive. To each his own, be loyal to a brand if you wish... But no two car makers are more similar than Audi and BMW, so if you love either one and can't at least appreciate the qualities of the other, you're either not being honest, you're blindingly loyal (for better t worse), or you're just silly (to put it politely). If you're blindingly loyal and don't shop around, that's you're prerogative. But the second place any BMW owner should look when shopping around for their next car should always be Audi, IMO.
Personally, I don't like driving the same car over and over and could never get 5 straight anything. I've had 3 BMW's and 2 Audi's, with a couple Mercedes and an Infiniti mixed in. I could easily go back and forth between BMW and Audi for the rest of my life and be very happy. They're similar enough to provide a very enjoyable driving experience, and just different enough to keep me from feeling like I'm driving the same car all the time.
Blonde, brunette, blonde, brunette. My wife wouldn't like that so much, but that's no reason my cars can't offer some variety!