It'd be unreasonable to assume that the 7 million that signed up (or even close to that number) plan on playing the game or even care about the game. I've signed up for a bunch of betas and never played them. There is still room to convince people to buy this game now that it has a $40 price tag instead of the speculated F2P model. Especially for those that signed up for a "free trial" and are on the fence about buying the game.
The streamers usually play the game for a few hours and never touch it again. I don't think you can create conversion from this but I don't think that we should avoid giving them keys solely because it doesn't create twitch viewers. I think twitch numbers will go up as the waves get larger. If we don't invite streamers because they provide little value, would you say that the average player generates even less value for the same playtime?