“What is wrong with presumptions based on similarities? Don’t most teshuvos deduce from precedent? ”
You think it’s the right of any random CR poster to do that and use it as a basis to posken for himself differently than the posek hador? Even Gaon admitted that frum poskim haven’t come out in support of the eruv after Reb Moshe’s psak. Also, the population of Manhattan has grown by a magnitude of 10 in the past century. How could it make sense to bring proofs from Rabbonim of the early 20th century?
“There is no such common practice.”
This is why I’m still posting on this thread. What issue do you guys have with just admitting that only MO people hold of the eruv? I thought I was pretty specific that when I referred to “common practice” I was excluding the Modern crowd. Otherwise, when would you draw the line? Why not include the millions of Conservative/Reform Jews in your “common practice” calculation and say that “there is no such common practice of keeping Shabbos at all?” The point is, on a theoretical level, you could stay and debate it all day; on a here and now reality level, it’s a sectarian shittah specific to the MO, and you need to stop pretending it’s any more than that.