Isn't it the case that in the gemara even in the situation of Yerushalayim at that time R. Yochanan was referring to a situation where even the construction of the city would determine whether it was a reshus harabbim or not? Certainly it was not being argued that 600,000 people would be walking through on one road but that a reshus hayachid could exist even if there were 600,000 people there?
DG
1) If we look at the Mishnah in Eruvin 59a we find that an "Ir Shel Yachid"; "a city of an individual"; is referred to. Rashi explains that this is a city where 600,000 people do not enter continuously, and therefore it is not considered to be Reshus Harabim.
2) It seems that the simple way of understanding this, would be to say that Rashi implies that if 600,000 did enter it always, it would not be considered an "Ir Shel Yachid" but rather an "Ir Shel Rabim"; "a public city"; which is a Reshus HaRabim, and therefore it is impossible to make an Eiruv there.
3) In addition, it appears from Rashi that even if 600,000 do not actually live in the city, but enter the city continuously from outside, this makes the city into a Reshus HaRabim. The simple understanding would be that if 600,000 people actually live in the city, the whole city becomes a Reshus Harabim, not just individual roads.
4) In addition, we find an opinion in Rishonim that even if 600,000 were not there at any one time, it is considered Reshus Harabim since 600,000 have access to this place. This opinion is cited by the very long Biur Halacha that I cited above; 345:7 end DH v'Y'A. This is from the Chidushei HaRan in the name of RA'AH. He writes that even though Rashi's opinion is that it is not a reshus harabim without 600,000 this does not mean that one requires 600,000, but rather that a large number of people, in the realm of 600,000, are accustomed to go there regularly.
5) It seems according to this that it cannot be a reshus hayachid if there were 600,000 there.
KOL TUV
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