The Earlier Sources for the Position That There Is No Reshus Harabim Today
The position that a full reshus harabim is absent in later times is connected to demanding conditions in the definition of a Torah public domain.
Why a full reshus harabim was considered rare
Many medieval and early modern towns were walled and gated. Their streets could be narrow, crooked, closed at night, or used by populations far below shishim ribo.
Under definitions requiring the combination of width, openness, and very large public passage, such cities often lacked at least one feature of the public roads in the midbar.
The position became part of practical halacha
Later poskim cite the absence or rarity of a complete reshus harabim when discussing the use of tzuras hapesach. This provided an important basis for communal eruvin in places whose streets were treated as a karmelis.
The position is a halachic conclusion about particular urban facts. It was never a declaration that the physical form of every future city could be ignored.
Modern cities require a fresh application
New York has populations, road widths, bridge and tunnel connections, constant access, and citywide movement that differ radically from an older walled town.
The earlier formulation must therefore be applied to the present streets and public. Repeating there is no reshus harabim today without that work assumes the conclusion that the poskim required a Rav to decide.