PAA: You’re basically saying that the concept of nishtaneh hateva proves that any metzius Chazal used to codify a halacha can have changed since their time, thus nullifying the halacha by simply engaging in a debate whether the metzius used changed? Where do you draw the line?
And how do you even determine what metzius Chazal used to codify a halacha and whether they in fact based it on a metzius or rather only used a metzius to explain a halacha that is in reality binding irregardless of the metzius which was only given as an explanation for a set halacha [i.e. isn’t given as a basis (causatively) but as a ta’am]? We don’t pasken based on ta’amei hamitzvos.