I dunno. personally, I'd find it kind of hard to say that making a let's play video is reproducing the video game. you could say it's reproducing some individual parts like the audio, some of the writing, and less concretely, the visual assets. but it's clearly creating a completely new piece of media where the game is not really the focus, and it is absolutely not reasonable to say that it takes away from the profitability of the game (outside of the possibility of the player sharing negative opinions about the game, which is irrelevant here), which is the entire purpose of copyright law in the first place
even the fact that it reproduces game audio alone is enough for the fair use claim to fall flat, but then you have on top of that copyrighted visual assets all over the screen constantly, and in many cases the entire plotline of the game (which is indeed copyrighted) being played out through a series of videos. I'm not saying it's impossible, but keep in mind the main categories for fair use is
1. Criticism and Commentary
2. Parody.
If you don't have significant commentary/criticism, or significant amounts of parody, then it starts to become really difficult to defend in court.
And I really, really don't want let's plays to become copyright infringement. I really don't. I hope that it stays the legal gray area that it currently is indefinitely and nobody ever enforces it, but we might be reaching a point where companies actually want to squash let's plays. Persona 5 did it, FEZ's Phil Fish
almost did it (he certainly wanted to), and now the Firewatch developer seems to be signaling intent to do it as well.
