Because I'm of the school of thought that as long as it is a nice car, you are happy with it and intend on keeping it for a fair bit why let it effect your decision to the point where you actively avoid certain cars for no reason other than the fact that it's going to lose a few more quid over the coming years.
Fair enough if you only intend on buying one to knock about in until F10 prices get more reasonable (2-3 Years?), but if you're going to keep it for the level of time you have had your E39 for or even longer I don't see the point of being so concerned. I can't see a 5/6/7/8 year old 330i/335i being worth anything like the £20k you have to part with right now.