Why not matrix? ( the “open network for decentralized communication”)

Is it possible? Sure. But few people are going out of their way to develop or set up a custom server to do that. And one might argue it's their right to do that! You can never be sure that you can delete information from someone else's machine, they could've just screenshotted the texts.

In fact, this whole article seems to be contrarion for the sake of it, with surface level complaints and no deep understanding of how federation and messaging apps work and their inherent limitations. Yes obviously you rely on the clients to maintain e2e encryption, but that's the case for every messaging app - even proprietary ones!