It all boils down to saving money. The airlines do not want to spend money to patch up a old plane that will be gone in 1-4 years, they'd rather spend their money elsewhere, like towards a new A/C for example.
Granted the cost fo a new airplane is by far more than the cost to repair the cabin of an old one, but if you neglect even 40 of those old planes you've saved quite a bit, because repairing all of them would add up.
Besies, these airline ar elarge enough to get away with that, they are well known, make tons of money, and ARE aquiring new planes right and left. So letting their image slide in one place should not hurt them too much.
Even those of you who flew on the NW 727s that were in bad shape will probably fly Northwest again anyway, right? so there's not much harm done to the airline, which in turn means they could care less the shape of their old plane because they still have the customers coming in.
Now if a new/startup airline were flying THEIR planes in that condition, they would go bankrupt rather quickly, because no one would fly them, not with that image.
Amazing the difference.
I wish we had Pan Am back.