1,
How do you define “greatness”? And how will that effect halacha? Moreover, as mentioned above, eruvin is not a new concept, and there are many precedent teshuvas from many great gaonim prior to Rav Moshe that clearly did not acknowledge Rav Moshes psak or method.
Poskim like the מהרש”ם, who actually was matir an Eruv on the east side of Manhattan, based on the info provided, despite being one of the most populated cities in the world at the time. (See vol 9 response’ Marsham).
Rav Chaim Ozer allowed an Eruv in Paris (see vol 4 אחיעזר), the Vaad Rabonim of Warsaw permitted an eruv in Warsaw, despite having more than 600k inhabitants, the reason was based not on Rav Moshe’s psakim. (See Tikun Eruvin of Rav M Kasher a copy of the letter written).
The דברי מלכיאל permitted an eruv in Odessa, despite having more than 600k (see ibid vol 4). The reason given does not coincide with any of Rav Moshes final psakim.
All these psakim do not fall away because Rav Moshe ruled otherwise. Every Rav can choose any of the above after thoroughly going through all written on the sugya.
Rav Henkin and Rav Yonoson Steif also permitted an eruv in Manhattan. I doubt they retracted all their psakim after Rav Moshe’s ruling.