I don't believe there's ever been a situation (at least in the USA) where there was no threshold at all. So the argument is over its value, not its existence.
That said, sure, you're right. But why are you right? I would suggest it is because we live (in the USA, among other places) in a culture that strongly emphasizes the right to pass along generational wealth. But this is not universally true across time and space, and our culture took a different tack (say, by quoting august Republican figures from the late 19th and early 20th centuries), the popularity or otherwise would likely be entirely different.
The argument is about the exclusion threshold because that determines who it is applied to. People generally support taking from other people but not from themselves.
A flat 50% rate still extract much more value from the rich, but apply equally to the poor.
My perception is that hereditary wealth transfer is about as universal and it's phenomenon get when it comes to humans. Not 100%, but close to it.