People are just putting up all the wrong points.
Reviews, guides do not showcase the entire game, while Let's Plays simply does that. Speedruns, multiplayer games have unique experiences thanks to the player's skills, and the people he is playing with. Let's Plays includes the non-interactive cutscenes of the story.
The issue isn't about ethics and what's good for the community. If a player can decide to stream a game for the entertainment of others, he can do it for free. The problem lies in when somebody is getting payed when a person watches an entire game that a game studio developed. Should the player gets it, should the developer gets it, or both?
The impact of sales for a game developer due to viewer watching their game can be good or bad, just like piracy does. However, it is also possible to get the viewer excited simply by showing the gist of the game (a review), not showing the entire storyline, or revealing surprises.
How much work did the person do to 'create that content'. The length of playing that game and speaking (the same length normal players you and me take to "create the content"). Editing work (It can take very very long). Someone with such a unique skill level that viewers can only admire him.
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The man hours a studio put into creating that game, definitely at least a hundred times more than the person who created that game.
I've never bothered with the Let's Plays side of things, people who say haters are jealous, so are game developers jealous of Let's Players? That maybe "Man, look at him getting money just playing our game, I'm jealous of him because even though we created all these content, people are paying him instead of us. It might not be so bad if he's doing it for free."
The day GTA V got released, Let's Play videos have already popped up. How do you feel if you were the developers, letting people watching the game for free that you've worked for years, storyline painstakingly crafted, surprises getting revealed.
Or how about this, someone created a video of himself watching a Let's Play video, and commenting on the Let's Player. Then he decided to monetize it, because he added to the commentary. Does this go well with people?
Maybe a Let's Player can upload a documentary of how a Let's Play is created, showing the amount of work involved, and we can get more appreciation for how much work it takes to create it.