Is that true that web forums tend to turn conservative over time, then?
People get more conservative as they age. Ten years is a long time; if the same people on the same forum stay for that long there is bound to be a shift in politics. People I know on the "right" have become much more centrist, and people I know on the far, far left have started to moderate themselves and move toward the, I guess "not no wingnuttery-left" is the way you would call it. Not exactly a move to the right but more of a move to "I have all this life experience now and can see other people's opinions in a non insulting manner." I'd be a lot more interested in seeing if there has a been a significant "staying around" of the old farts with 8-10 year old accounts on reddit. I bailed, deleted all my comments, killed my shitposting account and know of at least a good dozen other people I met when I started doing science outreach that don't bother with Reddit or deleted their accounts.
This is how I always heard the quote, although I appear wrong The thing I have noted in my life is that the radicals moderate and move to the center as they age. Black and white issues slip into veils of grey. Hard line positions that paint the enemy as an 'other' crack a bit as you meet more and more people. The exception to this rule is insular communities with minimal interactions with the out groups. This is also why religious and cult leaders are so desperate to keep the flock from being influenced by the outside. Hard to paint people as evil/other when you work and interact with them. There are dozens of ideas I had at 18-20 that came crashing headlong into the wall of adult reality, meeting people who had lived an experience that might have well as been in a movie. "Maturation" is not the word I think we want to use here. I think that "experience" is a better fit.If You Are Not a Liberal at 25, You Have No Heart. If You Are Not a Conservative at 45 You Have No Brain
Discourse will obviously be swayed by the inherent assumptions of a culture. The more participants are uncritically internalising those assumptions, the more those assumptions will become taken as unquestionable truth. Most people do not question their baseline cultural assumptions, so when "most" people join a forum, why would the discourse not automatically turn more and more into an echo chamber? This assumes that our culture is inherently right wing, which I personally believe is the case for the US. I've however never lived there.