Forums were also dedicated to "mostly related" things (like a subreddit) and it was a bit of effort to create a new account on a different forum, and you were obviously new at that point.
So "brigading" and such weren't really much of an issue (and the issue has always been people barging into a community, knowing nothing about it, pontificating on some post/issue, and promptly leaving).
Forums still exist, but you need a slightly strange sort of person to continue to put the effort into them when Reddit is "free and easy".
There are lots of solutions to needing to create an account now. There's social logins, and the prevalence of password managers if you want to use an email and can't remember if you already have a login.
Reddit is easier because you already have an account, but creating an account on a random site has never been easier to do or manage, so it's not that much harder (and importantly, people already do it because not everything is on reddit so it's not "new" to learn how to create an account somewhere).
I wonder if online discussions will shift to Discord channels. Sure it’s another centralised “forum” but it does seem like a credible alternative - a lot of discussion already seem to happens there.
So "brigading" and such weren't really much of an issue (and the issue has always been people barging into a community, knowing nothing about it, pontificating on some post/issue, and promptly leaving).
Forums still exist, but you need a slightly strange sort of person to continue to put the effort into them when Reddit is "free and easy".