1. I am disputing that a minhag from 2000 years ago or even 200 years ago is binding in a society that simply doesn’t act that way. At any rate, this wasn’t really my first point. Point #1 was that as far as I know nowhere in shas do we have an explicit directive from Chazal that women have to cover specific body parts. We have hilchos kesubah which people are trying to infer such directives from, but interestingly enough the halacha itself doesn’t seem to be stated anywhere.
2. Then why would you not take the next step, and subject the entire idea of tznius to such standards? If, for example, the frum women of the area do not cover their legs, why wouldn’t you agree that tznius does not demand that a woman cover her legs?