As a <30yo that grew up posting, moderating, and administrating forums (when I wasn't on IRC or Teamspeak), the decline in quality pales in comparison to the uptick in spam over the years. Maybe it's just different audiences but the spam was eventually so bad I moved multiple communities off of forums that we'd used for years and years- and got at least an hour a day of free time back in return.

In my experience spam is easily the biggest contributor to the death of forums, most groups just don't have the resources to keep their heads above water.

I see people complain on platforms like Reddit or Discord about one or two spam messages getting through, I don't think they realize how good they have it. A channel I follow on YouTube just got a single spam comment which one of his followers responded to, and he made a 3 minute video about it.

Can you imagine making a 3 minute video every time a spammer registered for your forums?