Expect more of this kind of incident as Kalitta has acquired many ex-NWA -200F's....and these are very old airplanes, of questionable serviceability.
Perhaps you haven't actually seen or flown those airplanes. I have, recently, and they weren't in bad shape at all. Quite good, actually. One is the last -200 off the assembly line, I believe, and featured a number of -400 parts...not a bad airplane at all. Furthermore, they'll be making a number of upgrades.
Delta/NWA didn't park them because they were no longer serviceable. They were no longer flying the Classic, and elected not to continue as the sole remaining legacy carrier with it's own freight division.