I'm not saying that terrorism isn't bad. Obviously it would be upsetting, but to be clear: an order of magnitude more terrorism would be a 9/11-level attack every year. Even with that catastrophic level, NYC would still survive. I understand this is an emotional topic (especially living here) but it's impossible to pretend that terrorism is an "existential" threat.

Realistically, as cold as this sounds, a terrorist attack which managed to level downtown would not even pose an existential threat to the US.

I concur with your assessment regarding human lives. That said, I think it's also important to take into account the effects those attacks have had on American society, such as the reduction of civil liberties, the increase in surveillance, and the changes in US foreign policy which have changed the position of the US in the world. These are significant. What would effect would additional attacks have? Yes, these are reactions rather than direct action, but at this point they've been frighteningly predictable. One could argue that this is an existential threat to the US as we know it, even if it isn't existential in terms of human lives, or in the country per se.

That's a fair point, but it's not what people who talk about the "existential threat" of terrorists are talking about. They're usually the ones pushing for more surveillance.

More terrorism might change the nature of US democracy, but it will still be the US.