So calling them "simple" is an insult, but if I were to say "your argument is plain, basic, or uncomplicated in form, nature, or design; without much decoration or ornamentation" that would be okay? Call me old fashioned, and forgive me for sticking to this one example, but I'd prefer to have a discussion in a place where calling someone simple (gasp) doesn't get a second thought. That's just me, I don't expect you to agree.
You are comparing two different things. Calling the person simple is a direct insult. Calling their argument simple is debate.
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