Yerushalayim of Old and Brooklyn: A Common Thread?

Rav Moshe Feinstein zt”l, using Yerushalayim in the times of the Bais HaMikdash as his precedent (Igros Moshe, O.C. 1:139:5, 4:89, 5:28:15), maintained that an eruv should not be erected in Manhattan even if there is no halachic basis against establishing an eruv there. Rav Moshe maintained that an eruv was not erected in Yerushalayim in the times of the Bais HaMikdash because of the fear that people who traveled to Yerushalayim from all over the world would, upon returning home, erect eruvin improperly. [The Noda B’Yehudah, Mahadura Tinyana Kuntres Achron (siman 21) and the Tiferes Yisroel (Eruvin, 10:57) maintain that an eruv was allowed in Yerushalayim, however it was not possible to establish an eruv at the time.] Since Manhattan like Yerushalayim has many visitors from all over the world, Rav Moshe maintained an eruv should not be established in Manhattan as well. However, Rav Moshe agreed (HaPardes, 33rd year, vol. 9; Kuntres Tikkun Eruvin Manhattan p. 161, and Divrei Menachem, O.C. vol. 2, siman 4) that if the rabbanim in Manhattan would erect an eruv, the precedent of Yerushalayim would not pose an obstacle. How much more so would Rav Moshe have allowed an eruv that had been erected in Brooklyn, as he was not sure if Brooklyn was even comparable to Yerushalayim at all (Igros Moshe, O.C. 1:139:6, 5:28:15).