if you call everyone dude and a transfem person gets mad about it, don't get defensive. just say like "sorry, i won't do it again" and don't argue "actually it's gender neutral" or "i call everyone dude". even if you do, i guarantee she's heard that argument from someone who very much does not call people they see as women dude. i certainly have
same goes double for the word guy.
You're conflating is and ought. Descriptivism is only concerned with what a particular language is while everyone here is talking about what a language ought to be. Prescriptivism isn't in opposition to descriptivism in the same exact way is isn't in opposition to ought. When people shit on the Academie Francaise for prescribing some ridiculous word for "seat belt" that no one actually uses, people are actually shitting on a colonial institution that's out of touch with European French speakers never mind Francophones not from Europe. But there are plenty of cases where prescriptivism is useful. The easiest case is on the topic of slurs. If the marginalized community believes a given word is a slur, it doesn't matter if the majority of society doesn't recognize the word as a slur. The word ought to be considered a slur and ought to not be used regardless of descriptivist arguments to the contrary.