If Delta is moving away from 747s, and opting for aircraft lighter than 777s (which may still carry just as many people), it sounds like they're trying to save fuel by flying lighter. Some of the newer models may save weight by using composite materials, but I can't help thinking they're sacrificing some cargo capacity as well.
Maybe they're betting that cargo will continue to decline as a percentage of their total revenue, and choosing their future fleet based on that? I know not long ago Northwest had the biggest cargo operation of any US passenger carrier, but I think Delta retired all their 747 freighters pretty quickly after the merger.