For me, the biggest mistake he made was put the signup wall without an initial reading limit of 1-10 tweets. Tweets are used as citation by journalists in news articles because people use it as an official platform and many journalists use it to report on what’s happening to a fairly large audience of followers. This is only one example where the signup wall ruins it all. You can’t even go in to read a single tweet. However, look at Medium. I despise it on a personal level due to the quality reduction it caused, but at least it only shows the paywall after your 1sth/3rd/10th article. At least it provides a window to your website from the internet. Elon Musk has slain that dragon too now, and all the windows to look through have been boarded up. He has walled himself and all Twitter users in.