Your argument seems fundamentally flawed to me. Sure, the proposal on, say, New York City will only be useful for those living or visiting. But that's not really a small number. There are over 8 million people living in NYC and around 50 million visitors per year.
Let's compare that to a fairly successful beta site. Raspberry Pi SE is probably only useful for people who own a raspberry pi, or are considering buying one. As of a few months ago, there were 2 million total Raspberry Pis. Statistically, the site is doing pretty well. So NYC is definitely a big enough community to support an SE site.
That doesn't mean I think these sites will necessarily succeed. I have no idea, but I see no sense in concluding that before it's even tried. If the developers and community managers decide that this isn't the direction they want to go in, then that's their prerogative, but I don't think it would be appropriate for ordinary users to vote to close this as "not a viable proposal" until it's actually been tried.