The plan for Staten Island secession assumed that the state would force NYC to continue to subsidize Staten Island. Staten Island would get some of the benefits of NYC's business tax base without the burden of its poor, and would charge high rates to place garbage at Fresh Kills while getting city water and the use of the city jail for peanuts. Also part of the deal would be SI residents being allowed to work for NYC agencies, but not the other way around. Finally, NYC would be forced to assume most of the debt based on property tax assessments, not income. Since Staten Island already benefits from a deal which gives it lower assemssment than much of the city on the same value, the debt provision would lock that in.
Anyone would secede under those terms. Leave the poor of Browsville to pay for themselves. Manhattan could have the lowest taxes and best service in the city if separate -- why should they pay for the Bronx if Nassau and Staten Island don't have to.
On the other hand, if Staten Island were willing to secede on even terms we should let them. They'd be worse off, marginally, and we'd be better off, marginally.