It heavily depends on the game. There are lots of single-player games where the story is much better than the actual game mechanics. Or where watching somebody figure out all the puzzles is more fun than doing it yourself. Here, streamers and let's-players likely cause a decrease in game sales. In simulation and competitive games on the other hand streamers should usually cause more sales then they prevent.

As one less recent example, watching Half Life 2 with funny commentary is great, but I have no desire to ever play the game myself.