It's all really quite simple.
An old friend of mine, who presently has a rather senior position at DAL told me two years ago that....'DAL will never purchase another 4-engine airplane and if we merge with someone, and they have 4-engine airplanes, these airplanes will be gone ASAP.'
In the case of freighters, DAL will sell 'em all and then, if they remain in the cargo market, will lease from another carrier.
It's called, passenger airline economics 101, DAL style.