For posterity alone I will post what I know of Rav Moshe’s position. This is from learning the teshuvos with those who had some relationship with him. I have no idea how Rav Moshe dealt with precedence. I did the learn the topic from beginning to the end. And of course there are great Rabbonim who are far more versed than me in Iggros Moshe. I’m sure they learn it differently (Read, better.) than me, but of those that I know they all concur that the eiruv in Flatbush is not in accordance with Rav Moshe’s viewpoints.
There are two opinions of reshus harrabim in the S”A. (OC 345:7) The first opinion states that markets and forums that are 16 amos wide (…sic…) the second opinion adds that if 600,000 don’t pass in it every day it is not a rsh”r.
There are numerous differences between these statements. Rav Moshe insisted that the only machlokes is about this minimum number. Three things come out from this. 1) There is no clear answer on what the first opinion holds is the minimum. 2) There is no differences that are dependent on this one. 3) What makes a rsh”r is the use of the area and not is shape and size.
So it follows that there are three psakim. 1) Without 600,000 we can always build an eiruv. And that has been the minhag as well. 2) The poskim that worked out instances of the Mishkanos Yaakov lekulah are not to be relied upon. That wasn’t the minhag. 3) The problem of d’oraysa is not about the circumstances of the town as much as the comprehensive area.
Rav Moshe starts his teshuva dismissing using the Els as a mechitza. There is a long discussion about Manhattan being surrounded by good mechitzos and the only question being the the bridges coming over them. Rav Moshe divides it into three shittos. 1) The Ri that it is not included in the partitions. But it is not a rsh”r at all. 2. Tosofos that it is included. 3. The Rosh the partitions are not including on top of the bridge at all. There are several possibilities. It could be a rsh”r. Or not, but still need a door that closes. Or a door at each end. (As I recall, there is another Rosh involved here.) Possibly even locked doors at each end. Rav Moshe works out some kulos even in this opinion. But he ends of that it would need doors to the bridges and they should be locked. This is about half of the teshuva. Any refutation of Rav Moshe’s shitta would take place on this discussion.
Rav Moshe explains that it would further depend if the bridges are included in the city than they would all be rsh”r because of the city according to all shittos. But Rav Moshe reiterates that perhaps the bridges are outside the city and a tzuras hapesach would be enough even according to the Rosh.
There is also the opinion that even full mechitzos are not enough for an open area where the public gathers. Rav Moshe expands this opinion and then says his shtikkel about Yerushalayim. Yerushalayim was fully enclosed but yet had a rsh”r inside it. This seems to uphold this opinion. And it is clear that there were times that putting up an eruv in a metropolis is not a given. And this that the minhag is to be zealous about putting up eruvin that is because they understood that there needs to be 600,000 for a rsh”r. All eruvin that were put up were in cities without 600,000. And we have no minhag to be lenient about this. This is clearly Rav Moshe’s psak not to put up an eruv without precedence.
Any attempt to explain away Yerushalyim’s lack of eruv would be applied to Manhattan. An additional matter of the max size of an eruv is mentioned. This would have impact encompassing the bridges an extending the eruv indefinitely beyond the city. He tehn writes off only counting certain types of people for 600,000. Excluding cars and trains. And rejects the theory of needing perfectly straight streets. As is was never really about the streets in the first place.
Rav Moshe signs off that even with locked doors there would be a problem of reshus harrabim in Manahattan and even in Brooklyn. According to some opinions it would still be a karmelis. There is no precedence against these opinions. And even if would disagree, there would be a the issue with the bridges. And even if that would be settled, ther would still be an issue because some would think that they can make an eruv in Brooklyn. And even if they fix in Brooklyn there would be places that would not be able to, or not care to build a proper eruv. And perhaps for that alone there should be no eruv even in Brooklyn alone.
One would really wonder what Rav Moshe’s opinions where regarding eruvin in Brrooklyn.