Well in this case I don't think it will happen but what has NZ's run rate been this test across both innings. Up near 5? I bet you in the past (and I'll say as recent as 3 years), people would have said that's impossible. Gayle and Guptill just smashed 200s in a ODI tournament and I bet no one thought that would have happened.
I think current trends show that history is not as relevant as it used to be. That's why I take the position that I do. Do I think it will happen? No, however just cause not many other teams have chased 400 does not mean it can not happen especially when teams are knocking 500 with a little frequency.
Conversely, if NZ needed 450 off 100 overs, and McCullum decided to go for it and got to 100 in decent time, would you back him?