I've been saying this all along... Stewart is so friggin far from Manhattan, you could get to Philly international faster. The best option will be the express bus from city to check in curb. It's affordable and a lot more simple than taking a series of trains and taxi(s).
But the NYC metro will be a tough sell to fly USGL out of SWF. The Russians, if there is in fact nonstop service to LED, certainly won't hoof it from Coney Island/Brighton Beach out to Stewart when JFK is 10 miles away. Even with a connection somewhere in Europe, it's an easier overall trip. And on the flip side, who wants to deal with a 2 hour journey back to Brooklyn from SWF after a long haul flight??
So, putting all that aside, it sounds like the markets will be ones that can attract residents of the area closer to SWF. Northern NJ, Northeastern PA, Western CT, points in Northern Westchester county and beyond, maybe even as far as up to Albany. Those areas don't have much, if any, nonstop European service so SWF may provide that niche.