Regarding Did New York water pressure drop due to toilet flushing after the Super Bowl?:
Another thing to keep in mind is that a huge portion of New York municipal water isn’t served directly from the water mains, but through rooftop holding/settling tanks (since municipal water pressure isn’t sufficient to push the water past the third floor or so without extra pumping). Any mass “exodus” would just affect individual buildings’ holding tanks, and then it would take a little bit for the pumps to kick in, which would probably end up naturally staggering the load on the municipal water system.