Twitter's another monopolist media company with a bunch of debt that needs to drown a potential competitor. I don't see Substack being able to become a direct competitor because the newsletter business is too good, and having closed twitter-like spaces that don't allow organized harassment is a good way for newsletters to build community and conversations. Twitter will remain the public version, where constant harassment from the ill and the covert has to be filtered out like spam.

That being said, maybe the important things will start being said in private spaces and only end up in "public squares" via screenshot ten or fifteen minutes later.