There is no doubt the -400 carries "more for less" than the older jurrasic jumbo. However, when you take in the cost of aquiring a "gucci jet" vs, the older jumbo, the cost factor difference gets a lot narrower. I am sure that the Kallita classics and Southern jumbos are making them money, otherwise they would not be ramping up the fleets.
The FAA is making it a bit harder for the "aging fleet" by requiring more extensive inspections. That may be the death of the older aircraft. Maintenance costs can go through the roof. But the "old lady" can still deliver, and will, until the forces of economics finally catches up. Let's hope it is a long way out.