If you're partnered with Twitch you're not allowed to stream to other platforms. I think affiliates still can.
Twitch is the long term (currently) if you want to grow your brand and popularity. If you put the work in, it's 100% worth it over mixer. If you want a payday and want to coast you sell out to Mixer.
A lot of these smarters high-end streamers probably want to be able to parlay into New Media jobs after they're done vidya gamin'. Run podcasts or whatever the fuck for decent money with sponsors if they're famous enough. Sit down and talk for 2 hours 3 times a week for the same pay rather than slogging 8 hour video games at the will of Twitch chat 5+ times a week.
You are right about the current status quo; but don't you think it would be healthier if that changed?
Right now Twitch can make dumb ass decisions like their purity council because they think that everyone will stay no matter how hard that boot comes down on creators. I'd rather Twitch be in constant fear of pissing off the viewers because Mixer or even (god forbid) YouTube Gaming is gaining steam and threatening to take a chunk of the market share.
I actually wish there was some enterprising company that made a viewer that consolidated all the platforms into one searchable app and then heavily promoted it so that the content enabling companies would have to be very concerned about people not hitting their site specifically to view their streamers.