B”H.

It is a good question, but that is what the Rebbe writes. In other words, if one can indeed not enact an Eruv which would remain secret from the masses, then it just should not be done.

However, in truth, in the case of the Manhattan Eruv, it indeed was not that hard. The Eruv was relying on preexisting structures, and they by and large did not need to erect new structures.

As a matter of fact, while there had been for a number of years extended discussions with regards to the permissibility of the Manhattan eruv in the rabbinical journals, in the end of the day, for whatever reason the existance of the Eruv was kept secret from the masses (even if various shuls had to raise money for the salaries of the Mashgichim). In one minutes, it seems like this very policy had been enacted on the advice of the Rebbe. It was only a few years later that they officially made the existence of this Eruv public.

אנו אין לנו אלא דברי בן עמרם

When addressing any halachic issue, its never a wise idea to put it up as a national referendum…

When an issue in kashrus arises, do we all start taking shu”tim and halacha sefers and begin to debate the issue.. no!
Because everyone understands that when it comes to psak halacha, its up to a posek.

Now,
When we have an issue like the eruv,
Where you have communities that their rabonim agree with it,
People start toying aroumd…

Perhaps if i follow the ruling of a different communities rov, so im ok…

But anyone who knows basic halacha, knows that this holds no water.

There is a concept in halacha called “b’asra drav, halacha k’rav, ubeasra dishmuel, halacha kishmuel”
Which means, that by a certain machlokes between the two great amoraim, rav and shmuel, each one had the say over his city.
And if you lived for example in the town of shmuel, but traveld to the city of rav, you would have to go according to rav.
And if you continued doing like shmuel, you wrre breaking halacha.

So that is where the concept of a mra d’asra comes in,
The word mara in aramaic is master, and asra means place.
And so mara d’asra translates as the baal habayis of that place.

So if you are in a town where everyone accepted upon themselves a rov as their mara d’asra, when you are in that town you cannot do things publicly, that are in contradiction to his opinion.

So now,
The whole question about the eruv in shchunas hamelech, crown heights, all the pilpulim are irrelevant