I'm working on an alternate future setting (somewhere between 2070 and 2130) in which Southern/South Western Ontario, specifically the London, ON area, becomes a tech powerhouse on par with Silicon Valley.
Unfortunately, I've made the classic blunder of coming up with an effect without considering its cause first. I've decided that somehow this new tech powerhouse affects the population of New York City (est. ~10m in this future), dropping it quite dramatically to almost half that (somewhere around 6-7m).
When I came up with this "effect" I decided that the population change happened over the course of a few decades, but that might not be a realistic time frame, and I'm flexible on that.
I've considered possibilities such as New Yorkers emigrating to Canada's new Silicon Valley, but 3-4m immigrants from the same city over the course of a few decades seems like something that would cause Canada to heavily tighten immigration requirements.
I've also considered slightly hand-wavey solutions, such as a plague, or other such disaster, striking NYC and causing the city to be so undesirable that it triggers mass emigration. I dismissed these solutions because they seem very Ex Machina-esque, I'd prefer a solution more rooted in viable politics.
What event, political problem, or other feasible happening could cause such a population shift?