Dear Youdont,
Eruvin is one of the most learned mesechtos by kolleliet today.
There was much more to Rav Moshe’s stance than population by area. Your entire knowledge of Rav Moshe’s teshuva seems to come down to just the parts where the word ‘chiddush’ is included. It’s like learning the whole Shulchan Aruch in the old print, but just where the pointing finger was inserted.
Rav Moshe published how he understood the sugya. He paskened according to how the public would treat the issue. Puk chazi how Hilchos Eruvin is slowly unraveling.