@Neville ChaimBerlin
“From my understanding the Mishnah Berurah says a Baal Nefesh holds by this definition. The Kitzur definitely advocates for the stricter definition in line with the Rif, Rambam, and Rashba.”
There is no doubt that we accept the criterion of shishim ribo lechtchila. The fact is we know today, contrary to the Mishnah Beurah, that the overwhelming majority of Rishonim uphold shishim ribo. In other words the MB’s list has been superseded.
“I never really understood the 600K shittah (even though I rely on it). It’s always stated in books like, “in our times there are no reshus harabim” (books written before big cities like NYC). That just doesn’t really add up since it implies that in the past they DID have to worry about it, when back then it would have required more than half of klal Yisroel to casually walk down the road.”
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).