Neville Chaim Berlin,
“Saying that an area with over 600K people is a reshus harabim is definitively not a chiddish. The shittah of the Aruch HaShulchan doesn’t seem to have ever been accepted by anyone; he’s just choosing his wording wisely when talking about a previous posek hador.”
First of all, Rav Moshe says explicitly that one cannot argue that shishim ribo is conditional of a city. Since they had established eruvin in cities that contain shishim ribo. Therefore, he came up with his chiddush of three million over 12 mil by 12 mil. This chiddush is what Rav Moshe is referring to when he states that he is saying something mechudash.
Second, The Aruch HaShulchan, chiddush has nothing to do with it. Rav Moshe is referring to the fact that the Aruch HaShulchan maintains that only machneh Leviah was a reshus harabbim. Rav Moshe’s chiddush is predicated on the entire machneh Yisroel being classified as a reshus harabbim. Third, Rav Moshe admits that his chiddush is not mentioned in any of the Achronim (in fact, most Rishonim and Achronim clearly maintain that only machneh Lviah was a reshus harabbim). Fourth, the Aruch HaShulchan is not a chiddush, Rashi says almost the same thing in Eruvin 59b.
“We went through this same issue with Richmond: you can’t claim that Reb Moshe “would have” been matir a Brooklyn eruv. He was explicitly asser it. We don’t need to guess how he would have held based on other teshuvos.”
Those who learn teshuvos do project into them. That is why they are written. The metzious is that Brooklyn does not meet Rav Moshe’s criteria to be clasified as a reshus harabbim. Moreover, Brooklyn is encompassed by mechitzos.
Therefore, according to Rav Moshe there is no reason not to allow an eruv. Especially, since Rav Moshe wrote that he cannot issue a psak din barrur.
Please learn the inyan prior to pontificating.