You,
“The point being, that early walled cities required that all people enter and exit through a particular street. Consequentially, these streets where the reshus harrabim of the city. Rashi states (Eruvin, 6b) that the main roads of Yerushalayim and Mechuza were classified a s a reshus harabbim (read Rashi carefully).”
True, as explained by the Aruch HaSulchan (345) who holds that only towns that have that particular structure of one main street are to be classified as a Reshus Harabim.
In fact, if you look into Rashi Eruvin (59b) he says as the following:
אלא לאורכה – דרך עיירות להיות פתחי פילושיהן לאורכם ורה”ר עוברת מפתח לפתח וחלוקה לאורכה הלכך אין בני עבר הלז רשאין לערב לבדן ובני עבר הלז לבדן משום דהני והני דרסי בהך רה”ר ויוצאין ונכנסין דרך פתחים לכאן ולכאן ורה”ר זו מחברתם שכולם מעורבין בה ואסרי אהדדי
As you can see the typical structure explained by Rashi, in the times of the Talmud, was with one main street.