It just comes down to what the streamer wants and the direction they want to take their brand.
He's not wrong about the drop in viewers for moving to Mixer and how your can't buy a community. Right now Ninja has slightly more than 8,000 viewers. That was his typical viewership back when he was playing Halo and Destiny before blowing up playing Fortnite.
Not only that, seeing that the average user that doc quotes (1.5m vs. 50k) - if I were looking to get into streaming, I would be even more excited to try going on Twitch as now there are voids that could be filled that was left by those who have taken deals.
In fact, as people have left for Mixer, Doc's numbers have continued to steadily climb because users are consolidating around those who are sticking around.