There has been some natural movement of financial industry jobs outside of NYC for just those reasons. This may accelerate as technology makes the spreading out of industries more viable.
However, we're not talking about natural job drift. We're talking about a dramatic event (such as an embargo of NYC) that required a rapid move of the entire NYC financial and banking industry. That would clearly be massively disruptive.
What you have found is much of the 'back office' departments in financial services have been moved to Jersey City , Westchester or merely out of Manhattan. Primarily this has been done for cost savings but also to decentralize operations in case of catastrophic terrorist attack.
That being said, the heart of the banking and the real brain power and creativity in the finacial services secot will stay concentrated in one urban setting. It's simply too difficult to remove yourself from all of the component buisnesses that are concentrated in onc city.
I work in Atlanta my manager is in Wa his boss is in Atlanta, and all of our corporate data is in Tx. No reason the financial industry can't do this more than they do. If NYC actually separates, then the tax support from upstate will go away causing the politicians to bleed the corporations even whiter. (After all you can't cut spending) The corporate executives with any sense of responsibility to the stockholders will remove their operations as quickly as possible to places where there is less government plundering. NYC will go downhill quickly because it really doesn't produce much wealth it just brokers the transfer which is what I said that started this whole thing in the first place. Gotta say that I've enjoyed the discussion though.