Most of us from many years ago have had 10-20 years to grow up, get through school, establish a career, marry, have kids (or watch kids marry and have their own kids), and generally have a different perspective from 10-20 years ago.Yeah I don't think the Civ games themselves are a reason for the forum's decline - it's still a top-selling, blockbuster franchise.
The pool of posters in OT is shrinking at a slow but constant rate. I will say that while this decline sucks, the people that have stuck around are generally nicer to talk with than the wider pool that used to be here. This place could be pretty vicious in the past and while there are obviously still spats, overall it is much chiller than it used to be and has a more cooperative rather than competitive vibe. There is also much, much less mindless spam and posture posting now than in the past and more genuine debate and exchange of ideas.
There are people who had negative histories who are either both gone or one half of the feud is gone, and that contributes to the more easygoing vibe of the place.
It's not all roses and sunshine, though. There are still disagreements and feuds that can flare up into bad situations. What I appreciate are good-faith efforts to patch things up.
Mediation used to be a thing used here at times, about 10 years ago. Sometimes it worked. Sometimes it didn't. It only works if all three sides - the two opposing individuals and mediator - go into it in good faith, with the goal of at least a truce, if not achieving peace.
I wonder if some of the former activities might come back if we had an opt-in thread pinned in OT, as we had in the social group. I never participated in the avatar switch, but I did in a couple of other things. It was both frustrating and fun at the same time.