If Wall Street goes south (both literally and figuratively) NYC will still be able to keep it's head above water. Two reasons: (1) Tourism, which is something Upstate and suburban communities don't have and (2) the motion-picture/television industry. Being the cosmopolitan town that it is, NYC is a natural "back-drop" for almost any film scenerio, whether it be action, comedy or drama. Also, the entertainment facilities -- sports teams, theatre, ballet, opera, etc., etc. -- will continue to be draws for generations as they have been for past generations.
About those high-tech firms on LI: while many have picked up some of those laid-off by Grumman, et al, they put to work less than had been laid off and furthermore those firms you listed are based on the fragile day-to-day ever changing world of computer technology that is almost as predictable as a horse race. Those hired today could be laid off tomorrow. It's happenned before. It'll happen again.
Doug aka BMTman