Yitay,
I think that you are understanding tznius too narrowly. I agree, and I think that it is muchach from the sugyas that the chiyuv to cover hair is unrelated to “saar b’isha erva.” But that does’t mean that it doesn’t reflect an idea of tznius. It is just a tznius that is not related to hirhurim. It is a tznius about dignity (and would therefore apply to a bald head as well). Both you and Daas are too focused on hirhurim.
I obviously think that my pshat fits in better in the Gemara than yours. I also think that the difference in character between shok and saar/kol are clear in the nature of the limmudim. Saar and kol are limmudim based on posukkim that discuss tayva. Shok is based on a posuk that describes erva as a symbol of shame, not hirhur.
Look at the Radak on those pesukim in Yeshaya. The same posuk that the Gemara understands to be discussing the revealing of shok, also discusses the revealing of hair as a source of shame. Why then doesn’t Rav Sheshesh use the same posuk to dervie “saar b’isha erva”?
According to me, it is beautiful. Uncovered hair as a source of indignity does not need a posuk in Yeshaya because there is already such a limmud from the Torah. But the posuk in Yeshaya is understanding gilui shok and gilui saar as providing the same moshul.
Rav Sheshesh is discussing a different din. The din of hirhur, the tznius of kol and tefach b’isha, not the tznius of dignity. Rav Sheshesh and Shmuel’s dinim are necessary because they are not basar and absent these limmudim I would have thought they could not have din erva like tefach. K’mashma lan that hair and kol can be erva for hirhur if ones lives in a society where they are normally covered.
My pshat is not the simplest but I do think that it is the least dochek.
I thank you, because if not for this conversation I would never have looked up that posuk again and I would never have seen the Radak.