Saying you haven't encountered malware seems like a weak argument; a tech-savvy person (which you are, since you're here) will know not to download dodgy stuff and not to click dodgy links on emails or porn websites.

A less technically-inclined person won't know, and will definitely end up exposed to malware. That's just reality for Windows users.

The key argument is that Windows's built-in antivirus is quite solid now. I don't know if (and I don't think that) a modern antivirus provides any -significant- additional protection over what comes with Windows by default.