“And what did the Gemorah have that we do not?”
Modern scientific methods. That is in fact the reason this thread was started — Chazal and Rishonim would never have asured tap water. It is a machloket whether modern science may be used to overturn a decision of previous generations.
“Surely you are aware that each generation (i.e. Tannaim, Amoroim, Rishonim, Achonim) could not argue with an earlier generation category (i.e. an Achron can’t argue on a Rishon)! This is a basic fundamental point.”
No, it isn’t fundamental at all; in fact it isn’t true! Among the Ashkenazic Acharonim who have argued on Rishonim were the Magen Avraham, the Vilna Gaon and the Baal HaTanya.
And Rambam argued on everyone.