Presumably this would be reflected in Gab, Voat and Parler being entirely normal discussion forums where people talking about storming capitols or Jewish problems would soon be shouted down by the majority userbase of perfectly normal people. Has this been the case?
The only one of the three that reached the point of having enough users to plausibly get there was Parler, and they were forced offline almost immediately after that happened. On the basis of a list of misbehavior which was unlikely to have been representative.
They had 10 million users. I have a hard time believing that there are 10 million "insurrectionists" in the country and if there were we have bigger problems. Meanwhile the cherry picked examples weren't anything you couldn't also find on Twitter or Facebook.
The network effect is strong. That's the problem to begin with. If people would quit so easily then there would be a lot more platforms and competition and we wouldn't be having this discussion.
And Slashdot has a moderation system that hides that stuff.
It's not quite true though.
It's what happens when you're the new kid on the block and the incumbents are appointing Witchfinders General to burn the heretics, because then rather than languishing in obscurity as you would otherwise, you get a huge influx of refugees and thousands of them are witches. Then if you let the witches chase off all the other refugees, you reach the undesired end state.
The way around this is to get big enough that most of your users are not witches. So the incumbents can do it already. Just stop having witch hunts. The witches can't take over when all the witches in the whole world are 0.5% of your user base, and other people only leave once the witches take over. It's a stable equilibrium.
But you can also get there another way, which is when the incumbents are hunting witches so hard that 95% of their refugees are innocent people. Then you just sit there and let everybody in, and your percentage of actual witches isn't so high that you're in trouble.
Unless anybody who tries to do that gets canceled before they can get a large enough user base to outnumber the witches.