Thank you for the great example. You've made your point in two sentences. Adding "That is nonsense" contributes nothing to your argument.
However, by labelling the idea as nonsense, by extension I must be someone whose ideas are nonsense.
This lowers the level of discourse. To what end? What did labelling my idea as nonsense accomplish? Did it help your argument? Or was it to make me feel bad, or to make you feel good?
Ignoring your last sentence, I do find the idea of ethics on rhetorical devices peculiar. Like - if I were to use a straw man device, would that be unethical? In a sense - maybe. I'd be misleading which is a form of lying.
I certainly wouldn't say labelling an idea as "dumb" would be unethical, though. It's just not productive to having a conversation: name calling begets more name calling. It puts people on the defensive where they are not receptive to new ideas.
Straw man arguments are a fallacy, not a "rhetorical device". It's an error we're all prone to doing at some point, unintentionally. I would argue that it's only unethical if it's done intentionally—and while you can guess at someone's intention, knowing is usually impossible. So unless you can literally read minds, only the author can truly know if they were being unethical or not.
Words don't exist only to "contribute to the argument" but also to summarise, to express emotions, as rhetorical flourishes, or simply to amuse. In this case, the last sentence is the conclusion of my argument. If you disagree, well, you disagree with my argument.
And with that you've just demonstrated my point. By focusing on an aesthetic component of my rhetoric rather than the substantive, you have chosen to steer this entire discussion away from the underlying topic.
> by extension I must be someone whose ideas are nonsense
That is nonsense. It does not follow.
> What did labelling my idea as nonsense accomplish?
It's my opinion. And while you're welcome to call that idea yours, I only consider it an idea. You shouldn't be so quick to tie your ideas to your identity. Everyone has lots of opinions and a good number of them are certain to be wrong.
Your argument is ill conceived, pedantic, and seeks desperately to justify previous positions.
But you are not someone who makes ill conceived, pedantic, or desperate arguments to save face. That would be nonsense. It does not follow. Don't be so quick to tie your identity to the pedantic arguments you've made.
However, by labelling the idea as nonsense, by extension I must be someone whose ideas are nonsense.
This lowers the level of discourse. To what end? What did labelling my idea as nonsense accomplish? Did it help your argument? Or was it to make me feel bad, or to make you feel good?