GAON:
“And as for the issue mentioned by youdont.. of the gezero etc. I don’t think its relevant after the Eruv has already been constructed. Thus, how can one say it is any transgression of הוצאה, because a גזירה מחודשת. The way I understand , it is a reason enough prior to withold one from constructing an eruv, but once its a kosher eruv, one cannot declare any issur because such a gezero.”
No I am not referring to Rav Moshe’s gezeirah regarding Yerushlayim (in which case Rav Moshe allowed once an eruv was established he would not be machmir; HaPardes, 33rd year, vol. 9). I am referring to the issue that Rav Moshe inveighed when he was told that that Brooklyn does not meet his criteria of a reshus harabbim. Rav Moshe argued (O.C. 4:88) that since Brooklyn is a large city and some my think that it contains shishim ribo, one should not establish an eruv (in any case, he finishes his teshuvah that he was then told that Brooklyn does meet his criteria of a reshus harabbim). In O.C. 5:29 Rav Moshe discussed why he would not make use of this gezeirah in Detroit, (because the eruv only encompassed two small areas), but otherwise he would have used it. Clearly one can make an argument that LA is no different than Brooklyn, in this matter.
Few people know about this gezeirah, but it would proscribe additional eruvin, besides for Brooklyn. However, I believe that Rav Moshe would not make use of this gezeirah if the area encompassed by the tzuras hapesachim contains a population less than shishim ribo. This would explain KGH, Queens. But this begs the question. the Brooklyn eruvin also encompass a population less than shishim ribo? So why did Rav Moshe oppose? The answer is that Rav Moshe believed that these eruvin did contain populations greater than shishim ribo (O.C. 5:28:5 and Addendum to O.C. 4:89). In fact these eruvin do not contain shishim ribo. So if we do not use Rav Moshe gezeirah in LA we can’t use it in Brooklyn, as well.