Dow and Durban

There are places where the sages changed the definitions required by Torah law, such as fearing endless waters instead of relying on a majority (I'm not sure I'm being accurate when I say that this was the reason for the permission), and more. I wanted to ask what the logic of the sages was to be strict in such cases, since for the Torah this was sufficient clarification. And vice versa in places where the sages are lenient against Torah law, why does it make sense.
I thought in several directions (I would love to hear the rabbi's opinion on them, and to hear additional suggestions), but it wasn't clear enough for me: A. Halacha speaks of standard realities, but there is an extreme case that cannot be included in it. It seems to me that this is a bad answer, firstly because Halacha goes down to the details and deals with distant cases, and secondly because the Sages have changed not only in remote and distant cases. B. There is a constant progression of reality, and things that made sense in the past cease to make sense (morally, etc.). This explains a lot of things, but to sum up the case of the referendum it does not, and certainly many more things.
In short, I would appreciate a good answer, thank you very much!