I don't know exactly how NY state taxpayers calculate their income tax liability to NY; however, I suspect that they "piggy-backs" their state income tax calculation off of 1040 AGI or 1040 taxable income. If that's the case, then TCJA provisions that lower either of those values will lower NY state tax revenue and it'll have nothing to do with whether wealthy people move from or remain in (as residents, proprietors, or employees) NY state.
New York state's rich folks overwhelmingly with regard to the totality of NY's wealthy residents, live in NYC, which is literally a stone's throw from New Jersey, which itself offers places for wealthy people to live. Also, Philly is a 45 minute train ride from Manhattan, which is where most of NY's wealthy people work. Lastly, NYC residents (rich and poor alike) are, unlike folks in most of the rest of the country, quite comfortable with using public transportation, so the prospect of having to take a train to work isn't a big deal.
Accordingly, NYC, the state's primary source suffers from the unique geographical circumstance of it being very easy for folks (frankly, of all income levels) to simply move. Moving won't obtain one who moves yet continues to work in NY any reduction in the taxes they pay to NY; however, they may find their own state's taxable income/taxes reduced by the portion of income or state income taxes attributable to NY.
The big reduction comes in moving from NYC to someplace other than NYC. The reason is that NYC directly imposes its own income tax. That it does may very well inspire folks to move to NJ, even though NJ is also a high tax state. Similarly, the federal cap on the SALT deduction adversely affects real-property-owning NY residents of all income levels, but especially in high income areas, because, in general property values are higher; thus property taxes are higher, even when NY's property tax rate is no different than the rate in lower property-value locales.
The point of the above is that the primary drivers to the decrease in NY state tax revenues can very well derive predominantly from federal income tax changes regardless of NY state's tax structure (including its various tax rates).