I really think we need some laws which require companies to open their protocols to allow seamless integration with other companies or open source implementations. There is no real reason why this should not be possible. E.g. take chat protocols. WhatsApp, Facebook messenger, Hangout, and all the others. It would be easy to have some way that they could communicate with each other. But companies don't want that, because they want the vendor lock-in. This is bad for everyone in the end. The only real way to solve this is if this is enforced by some law. I'm not really seeing that some law like this would be possible in the US, though. But maybe in EU...
You can't enforce that, but you can enforce that they open their own protocol, and also that the API is open to be used by anyone else, i.e. anyone can develop an own client.
How does that work with top-down moderation? E.g. some user X is harassing other users, so the company running the platform wants to ban them. If the protocol is open, isn't banning basically impossible, since they could just change to a client not controlled by said company?
edit: hmm, I guess this depends on what it means for the protocol to be open vs the platform itself...
That's why EU is so important. Its the only institution with even a chance. The US is far to captured to ever start thinking about basic anti-monopoly regulation.
Because that's all you are describing is: basic antimonopoly regulation.