I'm genuinely failing to see the downside here of facebook, twitter, and the like ceasing to exist.

Me neither. I should have been clearer. Despite all the bad things they do to make things worse, the problem is not the existence of the platforms. The problem is people. I was alluding to the fact that if there were enough people who recognized the problems of these platforms and acted on that, those kinds of platforms would never have arisen in the first place.

The various nasty types have always found ways to spread their messages, convert people to their cause, and convince others to do the actual dirty work.

Throughout history, every time a technology was introduced to increase the speed and geographical distribution of a message, extremism founded on false conspiracy, propaganda, disinformation, and misinformation has at least temporarily increased. There are really simple explanations for why that is. First, we have the problems of human cognition. Our brains are really lousy at identifying cause and effect, separating meaningful patterns from useless ones, and creating and maintaining accurate memories.

Second, truth requires verification. Verification cannot happen without investigation and communication among investigators. This means that verification will always happen much slower than message distribution. That is why a lie can circle the globe before the truth can get out of the starting blocks.

As bad as these platforms are, it's important to remember that their problematic algorithms are little more than codification of the methods that propagandists have used for centuries. Rush Limbaugh brought these concepts to a peak before most people had ever heard of the internet. Usenet was filled with the same stuff we see on Facebook, and there were no algorithms or central systems, just people doing what people do.