It's mainly a cost- saving measure, as the new paint is (from what NW
says) so much cheaper to apply AND lasts an extra year. So, it stands to reason that they would leave all else as is, to maximize the savings.
If you think about it, they say they get five years out of their current scheme. Which means that the 753s that have already been delivered (save one) and lots of A320s will be in the old scheme for five more years! Yay! I always like to see the old mixed with the new... it gives you a sense of forward movement within a company.
I have to say, though, that TWA did it wrong... they took far too long. Even after they merged with AA
, there were still planes in the old scheme...