That's why you build a personal brand while you're riding the coattails of a popular game. See Day9, who plays whatever game he feels like playing on his scheduled "day off" and streams it. He definitely has fun, even if it doesn't make him money (he recently streamed Mario, for example, which Nintendo [probably] claimed the revenue for) he's still building his personal brand and personal audience. His fans will follow him no matter what game he starts playing, because they're following him. But you're right that it's a hit-based system and if you don't become a hit (which requires a lot of effort and a lot of luck) then you're going to starve trying to "play video games for a living."
Yup. It's a new "field" if you will, and a lot of people are getting hurt during the birth phase. That said I've noticed streamers focusing more on being game agnostic, they'll demo new games and take note how much of their fan base follows. If the new game is a success they'll keep at it, if not they'll migrate back.
I don't think anything is different about playing Nintendo games on Twitch.
There's a streamer (iateyourpie) who makes a living from mostly playing Mario games, and usually not even being a speedrun WR-holder, just being an entertaining person who markets himself well.
