Since the PHL-TLV flights are so often jam-packed full of revenue passengers, there is obviously a demand for the service outside of the O&D of NYC. Maybe not enough for a second flight, though, and a second flight might very well run with much lower load factors.
But the money made between PHL and TLV is below deck in the cargo compartment. The real question is whether or not industry (especially pharmaceuticals) demands yet another flight between TLV and PHL. Bottles of pills and vials of potions do not care one whit whether they arrive at the loading docks of Queens or Essington. As long as they can efficiently make it to the various warehouses of Big Pharma, they are happy. My guess is that it is just as easy to truck (or fly) any of that stuff out of PHL as it is out of JFK. It's probably easier. And if one is using land-based transport, MIA is a nightmare.
Speaking of MIA-TLV, there seems to be a misperception that: 1) there are enough Jewish people living in the MIA area to make that non-stop profitable, and 2) only Jewish people travel to the Holy Land. (I use the term "Holy Land" because it is "holy" to all three of the Abrahamic religions...which has probably been the main reason Jerusalem has not been terrorist-bombed into oblivion.) I doubt that there is enough high yield passenger traffic in the MIA area (or willing to fly to the far corner of the US for a connection) to support MIA-TLV (which El Al has proven.) Tourists (Jewish, Christian and Muslim) might be able to fill a MIA-TLV flight on a fairly regular basis, but the low yields and lack of a belly-full of revenue cargo would not support the necessary TWO airframes to make the route profitable.
Yes, of course PHL doesn't have the O&D that NYC has, but it does have a huge geographical advantage over MIA for connections to Europe and the Middle East. (This is the reason, as explained by Scott Kirby months ago as the merger took effect, that PHL will remain a major hub for the airline. PHL's market is different from JFK's market, and there is room for both.)