Very smart of them to do this, though AS beat them to the punch in the last few days (though I can't imagine there is much overlap between potential JB and AS flyers... then again I am an example of an east coast AS FF).

Out of NYC I expect a lot of people to jump on this. DL and JB are the largest carriers here by a wide margin.

I have noted elsewhere that a lot of people who claim they will leave DL are going to quickly realize they have no choice but to take DL to their podunk destinations unless they want to connect in DFW/IAH/elsewhere. A lot of the smaller DL destinations out of NYC are not served nonstop by anyone (except maybe AA), though JB has grown in this regard in the last few years.

There's also the IROP aspect but there's no point in beating a dead horse on that. Most frequent flyers at this point know Delta is relatively reliable whereas JetBlue is not

-J.