You're right - there's an extra step. Most of the worst, most toxic shitholes on the Internet are not just unmoderated Nazi Bars, but actively moderated to silence the people telling the Nazis off. Nazi Bar explains the decay of USENET and 4chan, but not, say, Parler. Parler editorializes the shit out of their algorithm and website and actively bans left-wingers that might interrupt the perfect far-right echo chamber they want.
Major social networks don't do this, but they do actively boost things that create engagement, which is something right-wingers are very good at manufacturing. They use sockpuppet accounts to do that. This means that the underlying amount of toxicity on the Internet is not fixed - i.e. these forums not toxic purely because everyone else kicked them out and concentrated the toxicity, and we can't just disperse the toxicity. Dispersing the toxicity creates more toxicity.
Also, the forums you remember from decades ago were likely way more moderated than you remember, and probably run by people powertripping as hard as Elon Musk or Spez. The difference was that you could just leave without consequence and never come back. You don't have that luxury with Facebook or Twitter.
I think you're mostly being downvoted for tone but I agree with you to some extent.
To be clear, sites like Twitter actively went out of their way NOT to ban people for espousing far right talking points. Infamously there's the story about how auto-moderation would have resulted in the ban of a number of major profiles including GOP politicians and they had to give special treatment to Donald Trump while he was in office because he blatantly violated the terms of service all the time. Arguably more leftist accounts were banned than right-wing (if you exclude bans for active harassment like calling people slurs) but this is because leftists are generally outside the political spectrum so far right accounts tend to not be compared to leftists but to liberals, who of course tend to be more within the bounds of the terms of service.
The Nazi Bar is easily observable in real time with Elon Musk's Xitter. Elon Musk unbanned a number of far right accounts while actively engaging with a number of far right influencers. Additionally bans for slurs and targeted harassment have gone down. In Germany the only way to report an account is via a special flow that tries to implement the requirements of the NetzDG law (with malicious compliance in my opinion) and it's an open secret that ever since the layoffs any NetzDG reports sent via this system now get rejected by default whereas any sent by physical mail get approved by default. A German research project has also demonstrated that German-speaking left-leaning and far left "bubbles" have dwindled since the takeover whereas far right bubbles have gone into overdrive.
All German-speaking tweets (xeets?) I've seen from leftist or moderately popular left-leaning accounts are now flooded with replies from right-wing trolls (or worse) with blue checkmarks, often posting things that are clearly illegal speech under German law (e.g. glorifying criminal violence or directly insulting people to a criminal degree) but are not taken down even when reported. To be clear: this is illegal behavior on the part of X and will eventually result in them being fined. Musk just doesn't seem to care as presumably the fine won't be any more ruinous than what the company already owes.
You can however leave Twitter without consequence. Major influences, brands and others have already done so any many "normies" have stopped going because they experience the same toxicity and look elsewhere. Twitter hasn't been that large to begin with, certainly not on par with Facebook. Twitter's success came from it being the place where celebrities and journalists and influencers can be interacted with directly and you'd get a glimpse into the mind of your favorite famous person or be able to see witness reports of major tragedies first-hand (which of course became increasingly attractive for people trying to spread misinformation for fun or profit).
Major social networks don't do this, but they do actively boost things that create engagement, which is something right-wingers are very good at manufacturing. They use sockpuppet accounts to do that. This means that the underlying amount of toxicity on the Internet is not fixed - i.e. these forums not toxic purely because everyone else kicked them out and concentrated the toxicity, and we can't just disperse the toxicity. Dispersing the toxicity creates more toxicity.
Also, the forums you remember from decades ago were likely way more moderated than you remember, and probably run by people powertripping as hard as Elon Musk or Spez. The difference was that you could just leave without consequence and never come back. You don't have that luxury with Facebook or Twitter.