I very much agree, especially with point #8. Communities, online and off, by default start out with little to no moderation. Moderation is added typically only when there are elements that poison the ecosystem, as you put it.
I co-hosted an in-person philosophical discussion group for over a decade. At first we invited everyone to join, then we quickly learned that some styles of discussion destroy good conversation, so we started moderating or even asking people who could not refrain from them to leave the group. It was painful to do, but also necessary to preserve the culture.
A while back I saw some study showing that banning the most toxic subreddits greatly reduced the number of racial slurs on Reddit as a whole. It is for these sorts of reasons that banning toxic users generally and Trump specifically makes sense for Twitter.