Not really surprising considering the news that came out recently about how much money that 'Ninja' guy made (I was surprised though).
My memories could be blurred on that, but what I recall of the very early days (I.E. about the first year or so) of Twitch was that it was mainly, if not only a 'place about video games, for gamers'. Now, granted, I wasn't a big fan of Twitch to start with back when it was new, since I thought that YouTube already provided me with more than enough stuff about gaming if I just checked for it in the search bar. Ironically enough, it was back then that I would have been surprised at how different Twitch was from YouTube (I.E. YouTube has always been broad and wide open to all types of content, while Twitch back then really was more focused on video gaming related stuff).
However, since the past two years maybe, perhaps a bit longer I have noticed a major shift towards multi-entertainment content. It's all over the place. Sure, I suppose it can be argued that it's "still" more focused on video gaming stuff than anything else. And I guess people go there to still look for that... maybe. But it cannot be denied now that Twitch is very different than what it used to be in terms of variety (it has more, but it's definitely not always about gaming anymore). We still have streamers playing games and we can still watch big gaming tournaments. But there's also "general" life crap that nobody cares about, from some random Joe Blow guy in some remote corner of the planet going to a restaurant streaming it, or some dude walking down a street talking about how he loves Rice Crispies in the morning; to the inevitable young women with large breasts using Twitch to show themselves off while wearing skin-tight clothing and making sure there's a really deep V crack right between them to keep on the views and donations steady.
It's a bit of a cesspool of anything and everything. I don't consider Twitch now any different than just zapping TV channels and stopping on the stuff I might be interested in. However, that means more general views from more people around the world, than just video gamers. Which in turns means more potential for views, ads revenue, and of course... donations. In the end I guess it's all about entertainment "for the user", whoever you are and whatever you happen to like; chances are you'll find it on modern Twitch (but you wouldn't have back maybe 4 or 5 years ago). So ultimately for someone to be a multimillionaire out of streaming on Twitch (or combining it with YouTube) shouldn't be too surprising anymore.
What does surprise me though, are the people (gamers, in the cases I'm thinking about) whom apparently mostly rely on their 'humble' streams (compared to the big ones) to live off of it. I mean, stuff like streaming... say... Super Mario Maker with like 400 views (max, if even that) for freakin' 9 hours in a row? The guy might have received like $50 worth of donations that day, not sure about subscribers or ads revenue (if there's any when you start like that). I just hope that the 'small ones' who also want their piece of the cake on Twitch do have Plan B, or an actual job behind the curtain, otherwise I legit don't know how they go by even on a monthly basis with that sort of revenue.