>You do the edgy trolling. You try to get better at being edgy by coming up with better and better arguments for the edgy thing. You start having doubts of "wait, this actually sounds like a good reason?". You have no one to actually seriously discuss the issue with because its outside the Overton Window (ostracisation or bans would be given in serious places if you entertained the ideas), instead you find only stupid strawman arguments. Years of not finding anything to beat those arguments gradually shifts your views.

How is this any worse than the feedback loop of extremism and purity spirals you see in upvote base communities?

It just seems like a different mechanism for the same thing. In both cases the overton window is moving somewhere stupid one witty and well received comment at a time.