Do twitch streamers actually contribute to sales in a meaningful way? (Nintendo doesn't think so) I know quite a few people who watch gaming streams eagerly. They do not buy jack shit, though. A colleague actually spent more hours watching Dark Souls than I did playing it. And I played it for 150+ hours.
Twitch is another effective advertising avenue. For Honor sold extremely well at launch due to a heavy Twitch investment in sponsored streams by UBIsoft. Of course, player numbers dropped significantly afterwards due to classes not being well balanced, but they've been constantly working on that and player numbers are going back up.
PUBG is another game that benefitted from Twitch streams significantly. They Are Billions sold a LOT of copies due to Twitch streamers (and to a lesser extent YouTube). Lots of games benefitted to a lesser or greater extent.
That said, some games are more suitable to benefiting from Twitch streaming than others. Also, if a game isn't good in the first place, Twitch streaming isn't going to do much to make it sell better. If the game is already something that sells a lot of copies, Twitch streams aren't going to do much to make them sell even better. Hence, Nintendo's first party games don't really benefit greatly from it, but it certainly doesn't hurt either.
Both Sega Japan and Nintendo Japan were very anti-streaming/VODing of their games. Both Sega America and Nintendo America were pro-streaming/VODing recognizing it as another viable avenue of advertising their games. Sega Japan has come around and is now embracing streamers while Nintendo Japan is still anti-streaming unless they can find a way to directly benefit financially from it (on YouTube you either give them a percentage of your revenue or they content claim your videos). Hence why you see some of it on YouTube, but streaming their games on Twitch is risky.
Seeing games played is an effective advertising avenue for me. I trust what I see massively more than what some stranger on a gaming website (IGN, EG, whatever) has to say about a game they played. My latest example, Yakuza games. I've read enough that I find the concept interesting, but not enough risk buying it. Seeing it being played and seeing how the game mechanics work and what the gameplay loops is, I'm now REALLY wanting to play the games and excited at the prospect of playing them.
That said, streamlining (dumbing down) a game isn't going to help (Destiny 2, for example) it sell more due to streamers playing it. Because chances are, Streamers will also comment on it, and you'll see it.
Battletech is a recent example of a very complex game that sold additional copies due to Twitch. Granted it was a niche game which meant not many streamed it, but thankfully one of the larger streamers (CohhCarnage) loves tactical strategy games and thus streamed it and took the time to explain to viewers (as well as them seeing it in action) how things work in the game.
Regards,
SB