I think it's wrong to say that STN couldn't support a JFK service, STN is the closest major airport to the financial district and Canary Wharf in terms of travel time compared to LHR and LGW. It's also located in one of the most affluent areas of the UK, with Cambridge up the road and IT businesses in particular growing there at a high rate, there is plenty of demand.
We're going to see major changes in the long haul market in coming years with the rise of long haui LCC's and new aircraft types such as the A321LR. My guess is that what we saw with the rise of low cost, high frequency european travel will happen with the long haul US market in particular. Frequent flyer clubs will become less and less incentified and demand will focus more on price and connivence. I am a massive believer in Stelios' theory on the ignition price within aviation, if the price is low enough, demand will create itself. Based on these reasons STN could easily support a NY service.
However the airport is less known the US travellers and has a bad reputation within the industry because of it's Ryanair LCC image and a history of failed long-haul routes for whatever the reasons were.
The airport will get a NY service eventually but I hope they're wise in who they chase for one as it has to be a success, the likes of Thomas Cook to JFK would be a mistake. A Norwegian 787 service to a MAJOR NY airport would be a good fit.
The truth is the airport cannot cope with any new carriers until work is done to check-in and the main departure lounge waiting area, both are extremely overcrowded. They should never of knocked down Zone J/K.