To be honest I would have agreed with you before but I must be getting old. My Rangey does everything so well, fits my Rottie, comfortable, tows my trailer with quads or whatever even in to a muddy field and does have enough speed for overtaking safely etc. I have thought over and over about what to replace it with:

RS4 too small and not really for towing
RS6 bigger but again won't take trailer in to a field really or dog down on to beach through a muddy track
Q7 I honestly think interior in my Rangey is nicer even at 6 years old
Don't like Mercs
Everything else is too small

Really the only thing I would want more is a Cayenne Turbo and that's simply for a bit more urge, I don't think they are as comfortable though so would be sacrificing that a little. Supercharged Rangey makes no sense to me, I really don't feel they are that quick, certainly no where near a CayenneT and the amazing thing about the Cayenne is that it also handles well for what it is, the Rangey really just has a more powerful engine shoved in to a standard chassis.

So for me a 4x4 with a big engine does make sense as I can't afford a second car to just sit and be a sportscar.

My point on the Q7 V12 was that it doesn't REALLY complete the role as a fun big 4x4 I don't think, it really isn't worth almost twice what a 4.2 TDi is.

Edit to say: Now that I have thought about what Rhys said, if I was in the position to afford the V12 I could have a 2nd hand 997 GT3(sportscar) and Cayenne T and still be at less money so could keep Rangey as throw the dog in car hahaha.