Throughout contemporary halachah, it seems that there are generally no more actual reshus harabim's anymore (except for certain places such as Brooklyn, NY). My questions are:
Indeed it seems like in the days of the Talmud public domains were quite common such that the rabbis of the time instituted many, many rules to prevent accidental violation. But on the other hand everything we know about history suggests their populations and urban areas were much smaller than modern ones.
Without getting too detailed, there are a few different broad approaches one could take to resolve this contradiction.
Modern cities have many more walls and fences than classical towns would have had. All the extra boundaries somehow formally divide the area up and define it as private despite its large population. (I'd call this a "Chazon Ish" position.)
The paradigmatic public domain is not actually a busy urban area but rather the open lawless rural wild. The extra rabbinic fences are not there to cover accidentally carrying in the city but rather to cover living in the city and forgetting that carrying outside the city is a problem. (I'd call this a "Rambam" position.)
Historians are heretics and the metropoles back then were actually bigger than current ones with fantastically busy thoroughfares. (I'll decline to assign the position to anyone.)
There are actually lots of public domains nowadays and most city eruvin are unfortunately invalid (albeit perhaps only rabbinically so). You say "throughout contemporary halachah" but there is no shortage of recent great rabbis who did not carry in most modern eruvin, and the widespread lenient practice is easily attributable to the obvious difficulty in following this position. Note too, there isn't really any particularly old tradition for being lenient in modern size cities since they didn't exist only about a century ago. (Let's call this a "Brisker" position.)
