You're not trying to convince them. You write to the lurkers--to the people who are curious about fanfic and the issues involved, and want to figure out why people do it and whether or not it's legal and so on.

You write *in counterpoint to* the people who've already made up their minds, but you're not writing *for* them.

If nothing else, you hope to convince the lurkers that it's a big complicated issue with a lot of aspects to consider, so they won't be so quick to accept it when some Famous Author says "fanfic is theft of my hard work, and that's the end of it."