Dear Youdont,
I’m not anti Eruv. The problem is that you are assuming what I am from an online post. Just like you assume Rav Moshe’s entire rational from Rav Dovid’s letter. Where in his teshuva on Williamsburg (I 138) Flatbush (IV 87-8) or Boro Park (V 28) does he write that Brooklyn contains three million people?
This is okay for the coffee room. But it is not allowed in bais midrash.