Regarding 747-200 freighters, even FedEx long ago discarded theirs, after they purchased FlyingTiger.
It is not surprising that Delta wants to rid themselves of these older airplanes.
Will they be used again?
Doubtful, in my opinion, as the freight market will take quite sometime to recover and....the 747-200 is simply too large for many markets.
747-400 freighter?
These will be in much greater demand on the freight upturn, but even these will be eclipsed eventually by the 777F.
You simply cannot disregard twin engine economics (versus 4-engine) in the longer term.