Maybe if your game is a linear pseudo-Movie that can be fully substituted by a few LP videos, you have something to worry. But that's not LPs damaging the game industry, that's the game industry churning out unoriginal and, frankly, lame games.
I watched an entire walkthrough of Batman Arkham City, but the combat system was so awesome/fun-looking that I'll eventually buy the game myself and play it (probably even go back and buy Arkham Asylum too, as it has the same combat system -- though an earlier version).
That said, I also watched through the entirety of Max Payne 3, and it didn't make me all that excited to run out and buy it. It mostly made me remember how the bullet time became less fun after hours of using it in actual game play (in the first 2 Max Payne games). In reality, it was just the story that would have driven that game for me (and in the end, I didn't really like the story that much anyways).