Actually, I think it is largely the fact that the more expensive a car is, the weaker the used car market for it tends to be, unless it's in short supply (e.g. some Ferraris). Used car buyers tend to be more price-conscious than new car buyers.
I also agree with Raleedy that higher operating costs play a role. Used car buyers will be more likely to want to minimize these as well.
Reliability, if anything, would count against the 535, as the E60 535 had a notoriously unreliable fuel pump. The new engine could very well avoid this problem. I hope so, because I conduct a car reliability survey and I'll be able to provide more precise results for the F10 if I don't have to split the sample by engine from the start.
About the survey--additional participants always helpful:
Car reliability research