Where did all the Reshus Harabim's go?

The Gemara in Shabbos has extensive discussions about what jewelry women can wear on Shabbos, what are animals allowed to go out with, etc. all gezeros in fear of the person violating the issur d'oraysa of carrying 4 amos in a reshus harabim. It is patently clear from all of these that in the the time of Chazal that they felt that this was a very valid concern (e.g. carrying in a reshus harabim) as we know that that Chazal followed a number of rules when making Gezeros 2 of them being:

1. milsa d'lo shchicha lo gazru bei rabanan - Chazal did not make gezeros for far-fetched cases.
2. we don't make a gezera l'gezera - Chazal only made gezeras to protect people from violating an issur d'oraysa, they did not make gezeras to prevent people from violating an issur d'rabbanan.

Based on the above Chazal would only have made all of these Gezeras regarding carrying if there was a real concern that people would violate the issur d'oraysa of carrying in a Reshus Harabim. What this means is that there must have been MANY places deemed Reshus harabim in existence
in Israel (and Bavel) otherwise they never would have made all of these gezeros.

However, this is very very difficult. Tosafos on 64b already makes the claim that we don't have a reshus harabim d'oraysa nowadays because we don't have 600,000 people and our streets aren't wide enough. This opinion of Rashi and Tosafos has been accepted l'halacha and is the basis of all city Eruvin. But, if this is true, then how in the time of Chazal did they ever have a Reshus Harabim d'oraysa? We know from population studies as well as simple logistics that pre-modern cities could simply not be that big. Without modern infrastructure there was simply no way to bring in enough food and water to feed a population of 600,000 people.  It is not believable that in the time of Chazal the population of the cities in Israel was larger then today. The only city today, in modern Israel, with a population over 600,000 is Jerusalem and only a fraction of that is in the old city (which is the only part of the city that existed 2000 years ago and in fact was smaller). Every other city in Israel today has a population less then 600,000 people. There is no way that in the times of Chazal the cities were anywhere near as large given what we know of the population etc.

Why would they make all of these gezeros if there was no Reshus Harabim d'oraysa?

The fact is that the Gemara talks about whether you can wear amulets or not on Shabbos a gezera that you will carry it in reshus harabim. The Gemara discusses whether you have to take the amulet off to go to the bathroom, if you do there would be a problem because maybe you will carry it. Rashi comments, the bathrooms were outside in the fields and therefore there was an issue of carrying in a reshus harabim. It seems clear that Chazal thought that Reshus Harabim's were all over.

In short, how could it be that in the time of Chazal 2000 years ago they were so worried about people violating an issur d'oraysa of hotzaa when we can't find a Reshus Harabim d'oraysa today? Brooklyn,

with a population of 2.5 million people (larger by far then any city 2000 years ago) is the only place that I have heard of that there even is a discussion about. There are Eruvin in every large city in Israel

(based on the presumption that the city is not a Reshus Harabim D'oraysa) and many large cities/communities in America. How do we reconcile this?

The answer clearly is that there never was a requirement for 600,000 people in a Reshus Harabim and that Rashi and Tosafos were simply wrong. What does this say about mesora and psak?

Thwere are some interesting related questions. When were the gezeras related to carrying established? The Gemara states that Shlomo was mesaken the need for an Eruv Chatzeros, however, what about all of these other dinim d'rabanan? When was the takana of carmelis made? When were

all of the gezeros listed at the beginning of the 6th perek of shabbos (regarding not wearing jewelry etc.) made? When was the issur to blow shofar on Rosh Hashana on Shabbos made? I don't know of any sources that discuss when these were made. It would be very interesting to know how early/late they were. 

The bottom line is that we see even on issues where there seemingly is a strong mesora, there isn't. Something as simple as the definition of a Reshus Harabim is the subject of intense machlokes.