SP games in which skill isn't the main draw are obviously hurt by lets plays. Common sense, right?
If you're making an FPS, then people will want to make those kills themselves. But if it's a puzzle or adventure game? Once you've seen the puzzles solved, why would you want to do it yourself? There's one solution, and you know it, so there's no joy (or point) in doing it yourself. A game like Myst or Monkey Island would be killed, as watching someone play shows all you need.
For those walking simulators, where the point is solely plot, it's like letting a Let's Play show The Terminator. Why would anyone buy it if they can see all there is for free?
Not saying the solution is stopping them, but there are absolutely genres where it causes lost sales.