It's tough to get lurkers (like me) to speak up and shift the discussion when the emotional cost of doing so - arguing, being downvoted, getting defensive, being ridiculed - seems to make the effort simply not worthwhile for the individual. Not only on the web, but also in the office, at Thanksgiving dinner, at a cookout, etc.
the 98% works in that case too: there are many people who might benefit from your comments but don't interact in any way, so you sometimes may deem the costs worth it (like now).
Indeed it does. And the "insane" label to the engaged is even more apt there.
In more controlled, personal settings my favorite response along that vein is "what factors did you consider that led to your conclusion?" It's a less offensive, more roundabout way of telling someone that their assertion is questionable that also betrays their thought process.