You're right that online debates between a staunch Lib and Conservative never change their stances.
But like you said, I strongly believe there are dozens of on the fence lurkers who will say "Ah HA!" to a calm bit of logic. That's how I was converted, so to speak.
By kind people who pretty much said, "this is a healthy way to view life and the world"....been there ever since.
Same here.
For me, being a conservative means that I can think for myself, that I don't have to follow the party dogma, that I can disagree with my fellow conservatives.
When I do argue with liberals, usually in a face-to-face situation, all I do is to plant a little seed of doubt in their heads, let them start thinking on their own. Eventually, they will walk out of the liberal plantation on their own.