I'm no expert on sociology. I think I see where your coming from.
Reality isn't just there. Shared understandings which change over time is how we construct our understanding of world around us.
Not having a go at you here but couple of points.
Stepping back isn't the argument here on use of Nazis as a metaphor sign of healthy debate?
As metaphors change over time?
I'd agree with
Rob Ray that using Nazi metaphor lays one open to attack.
The other shared understanding that is newer Id say would be the word Apartheid.
It's now used and widely understood as term for a state that practices racist policies against indigenous people.
If I say Apartheid pretty well everyone I know has some understanding of what that means. Even if they don't follow politics.
To say that Israel state is an Apartheid state. That practices Apartheid within pre 67 borders and in Gaza / West Bank to me is much better way to use a term than Nazi.
Apartheid was originally SA term. I think to build on that kind of popular understanding is much better than the frequent ( I see on social media) Israel are like Nazi Germany.
It still winds up the Zionist/ Israel supporters. But whether one says it a settler colonial state, practices Apartheid or is like Nazi Germany they are going to say you can't judge Israel like this.
End result of that is Israeli exceptionalism. It stands outside all these descriptions or metaphors
That I think is something to object to.