But even then, Matrix will likely not scale well enough to replace Twitch's backend, or to be able to nicely handle QuakeNet's event channels.
You'll also have to have a native client at that point, because the webclient also breaks down at those amounts. And you'll need to use a binary protocol or simplistic text protocol between client and server, because otherwise the user's mobile connection won't be able to handle that traffic.
Matrix will have to change a lot of things before that scaling is easy.