I don't think the DL JFK is in danger of the chop anytime soon, as far as I know DL are still keen on their JFK operations and that includes the European stuff. I think other European stations DL serve have also seen some tweeks or what some might term, "downgrading" of aircraft type too over the last couple of years, not just MAN.
I think its all more of a "fine tuning" exercise trying to get the best aircraft usage for each particular route. Also in the past the DL JFK has gone back up to daily in the summer IIRC so I don't think there is too much to worry about yet.