I'm sure the platforms manage their ethics, but this idea that we should trust them with moral authority because they deal with evil more than anyone else should be more suspect. I think it is far more likely that they are compromised by interests than resolute defenders of principle. At the end of the day, social media companies are just euphemistic peddlars of nuanced variations of porn, and a we probably shouldn't be relying on them for judgment on the quality of public discourse.
Censorship (to say nothing of the co-ordinated censorship between platforms) is a blunt instrument with a lot of side effects, and people are surely going to start up their own version of something, I'm saying we should think a lot harder about these issues, because we really don't want to be around when people finally get sick of it and start erecting their own platforms.
I agree with the solution though, which shouldn't rely on platform moderation whose customers are advertising and PR. The platforms won't have leverage against their own customers here.
This isn't nihilism, nihilism would be ignoring the problem. And on angle to tackle the specific problem of parties wanting to clean up the net in their purpose was the insistence on neutrality and laissez faire moderation.
I think it still is the best course but requires education like you suggested.