I'm sure there are solutions. The east coast is loaded with airports that Delta could use. My wife's trip to Bulgaria this year is going to Boston and from there to Amsterdam and Bulgaria. I'm not enough of a Flytalker to tick off all the alternatives, but I do know they are numerous. I'd be quite surprised if someone told me that foreign airlines connect NOWHERE but JFK. And that's probably what Delta wants and needs most, the feed to and from those other popular airlines. Granted that in some ways, JFK really makes is smoother. But if they really dislike their terminal, their technical staff should be able to phase replacements in over time. I think any solution has to fit in the Delta budget. They don't have truckloads of cash to fling about to benefit a particular airport.
(Do they?)