It's not just gatekeeping. Recall the "Eternal September" concept. Prior to "Eternal September", it used to be that every September, a new wave of college students got access to the internet, and they would misbehave. But eventually they learned the norms of how to behave online, and things settled down.

That suggests a plan for internet reform. Start with a small group of contributors devoted to a better internet. Let new people in at a slow rate, so they acclimatize to norms of how to behave.

Maybe strict gatekeeping isn't actually necessary, and it's just about (a) starting with something good, and (b) adjusting the rate of newcomers to ensure that they actually acclimatize. If your platform grows 1% monthly, that will produce rapid compounding, yet could still be slow enough for acclimatization.