I'm Not saying ISP absolutely won't happen (I'm sure if the incentives, price, and revenue guarantees were right, NK would open Hicksville, KY), I just think Islip doesn't follow Spirit's latest expansion model of operating to primary airports in large markets.

One NYC scenario I could see is NK swapping their LGA slots to B6 (or possibly a legacy) for some of their JFK (possibly EWR) slots. This would allow NK to expand their reach and fly to places like LAS and the West Coast from New York, all while keeping them in a primary NYC airport. JetBlue (or other legacy) would get some highly coveted LGA slots, slots that aren't necessarily more advantageous to a ULCC like Spirit but are highly sought after by a more business oriented airline like B6 (or other legacy).