1. Advertiser revenue dropped like a cliff in 2008 era. Facebook advertisements were far more valuable than AdSense whatever from your smaller forums. Today, you will never have the revenues of the 90s or 00s era, and likely never again as long as Facebook and Youtube have eaten up the bulk of advertising eyeballs and dollars.
2. Moderation back then was far harder. There was far more spam to deal with those forum-sites back then and moderators had to work harder than today. Yes, we got trolls and other such moderation issues. But 90s / 00s were just filled with penis-enlargement spam and other crap that no one wanted. If you didn't see this, it was because your moderators were excellent (and likely overworked).
For this "moderation" issue alone, I'm convinced we need shared-identity systems like Reddit or Twitter, where an admin-level ban can quickly clean up spam. We have "higher-level" moderation problems now, but at least we've evolved past the manual-labor level of spam cleanup.
> Lastly, I could get all the relevant updates from the sites I was interested in with a nice orderly rss feed customized to my liking.
3. Twitter beats RSS feeds in terms of advertising revenue. If you're aiming at an advertisement scheme, you ain't beating the "new web".
Without advertising revenue, you ain't rebuilding the 90s or 00s. And you won't get advertising revenue, because today's 2020+ era of websites are far better at drawing eyeballs than any no-name forum or phpbb instance.
Open Source projects can host a Discourse or other instance due to community donations. I'm not sure if typical groups on the internet today are large enough to self-fund (heck, even popular open source projects have issues self-funding their discussion sites or email lists).