Developers will react to this tax, but it's most likely they'll start building things that are a better fit for average households. And those pencil apartments? Likely they'll become cheaper. More will be purchased by residents. Those residents will still likely be very wealthy, but if they are true residents they would have competed for housing somewhere else in New York, so this will reduce the rates (if ever so slightly) of other property.
The real value however is putting developers focus back on building housing for a broader segment. If zoning is not so egregious as to prevent this then you'd see a more affordable New York than the alternative. A few extra tax dollars wouldn't hurt either.