I disagree. If you remove NYC, then upstate NY becomes a very competitive battleground state, probably with just a slight Dem advantage. If you remove the NYC votes from New York State's 2004 totals, then Kerry would have edged Bush by only about 93,000 votes, taking 51.1% of the two-party vote.
With the NYC votes included, Kerry won New York State by more than 1.3 million votes, taking 59.3% of the two-party vote.
Psycologically, by severing NYC from NY State, it would allow for much more conservative politicians to run in NY State. It would be a battleground state no question, but would become something between PA and OH.