Rabbotai,
We are told that Takanas Ezra that a Baal Keri must go to the Mikveh was Misbatel because it was a Gezeira sheyn Hatzibbur Yachol L'aamod Ba, it was impossible for everyone to keep. Yet we find in Berachos that the Tannaim and Amoraim are still discussing and arguing its applicability for centuries after Ezra. How long does it take to establish whether a Gezeira stays or is not applicable because the tzibbur can't fulfill it? Is there a trial period? Are those who don't keep it at first Oveir on not listening to the Gezeiros of our Chachamim?
Samuel Kosofsky
There are clear-cut cases in Shas (such as that of Shemen Akum) which were not accepted by the people and which were therefore negated later. This is not one of them. Rebbi Yehudah ben Beseira maintains that Takanas Ezra was later negated. But not all the Tana'im agree with him. Then the Amora'im argue over the Halachah.
In addition, there are opinions who hold that although the Takanah was canceled, that was only with regard to Torah, but not as far as Tefilah is concerned. And then there is a Machlokes over whether Tefilah requires Tevilah of Nesinas Arba'ah Kabin.
Kol Tuv,
R. Eliezer Chrysler