“As per Chazon Ish, most cities nowadays are by default a reshus hayachid”
To be clear, this is the Manhattan/Brooklyn-eruv-supporter interpretation of the Chazon Ish, right? Nowhere is there written evidence that the Chazon Ish called downtown Manhatten a “reshus hayachid.” I’m aware of the shittah to which you refer, and it’s your right to interpret it as you’ve been taught, but it’s a little disingenuous to make it sound like there’s an explicit Chazon Ish which states as you stated.