R' Feldman writes
In countless places where the commentaries, whether Rishonim or Acharonim (Early or Later Authorties), are faced with a contradiction between the science of their times and a statement of the Sages, they commonly apply the principle, nishtanu hateva’im (“nature has changed”).1 Had they held R. Avraham’s view, they would have simply explained that the Sages erred in following whatever was the medical or scientific opinion of their times.
The premise of this proof is that saying that Chazal made a mistake in science is a simpler and more compelling answer then nishtanu hateva’im. Otherwise there is no proof, the Rishonim answered nishtanu hateva’im because it was a better answer. Therefore it must be that saying that Chazal made a mistake in science is a simpler and more compelling answer then nishtanu hateva’im.
This gives us an insight into R' Feldman's thinking. Even according to him it is more logical to say that Chazal made a mistake then nishtanu hateva’im.
Now we need to ask is his premise correct? For us living in 2005 it certainly is for the following reason. On one hand, we know that the science of Chazal's time was bogus and completely wrong, on the other hand we know that nishtanu hateva’im is difficult from a scientific perspective. therefore to us it makes much more sense to say chazal made a mistake.
However, if we look at it from the Rishonim's perspective the picture changes completely. I believe that for the Rishonim nishtanu hateva’im was a more compelling answer then saying Chazal made a mistake in science for the following reasons:
1. The science of the Rishonim was the science of Chazal. Science progressed little if at all from 300 to 1300 and in Christian countries may have regressed. Therefore, the Rishonim would have no reason to doubt the science of Chazal, they relied on the same science and they believed it was true.
2. Nishtanu hateva’im made sense from their perspective. The rishonim did not understand the world from a scientific perspective and they did not see the difficulties inherent in saying nishtanu hateva’im, in fact, it probably fit in with their world view.
Given the above, Tosafos Moed Katan 11a when faced with the statement of Chazal that rotting fish is healthy vs. the reality that it is not had 2 possible answers:
1. The science that Chazal used was wrong
2. nishtanu hateva’im
I believe that from Tosafos's perspective the second answer is more compelling. They had no reason to doubt the science of Chazal, it was their science.
R' Feldman writes they would have simply explained that the Sages erred in following whatever was the medical or scientific opinion of their times. Than answer to this is very simple, the scientific opinion of Chazal's time was by and large the same scientific opinion of the Rishonim's time.
R' Feldman projects the attitudes of someone living in 2005 who knows that the science in Chazal's time was wrong, to the Rishonim. However, the Rishonim had a completely different perspective on science. They had no reason to assume that the science that Chazal based themselves on was wrong.
I will take this 1 step further. As I pointed out, to say that Chazal's science is Torah M'Sinai on one hand and nishtanu hateva’im on the other hand is very very difficult (see Is נשתנה הטבע a viable answer for conflicts between Torah and science?). Therefore I would suggest the opposite conclusion then R' Feldman, namely, those Rishonim who hold nishtanu hateva’im hold that Chazal did not get their scientific knowledge M'Sinai and therefore they could say nishtanu hateva’im.