Who's in charge of Twitter? I'm going to go with Twitter.
Twitter is used for formal announcements because Trump himself has decided that it is a good place to make them, not because Twitter has placed themselves in that position. Granted, they make like being in that position and it probably benefits them greatly due to the increased ad revenue and engagement, but they can also make the decision that his presence is not worth the vitriol it generates. He can easily choose another avenue (Facebook, Reddit, News media, etc) to make announcements if it is no longer the best choice for him.