There's plenty of nuance here. For example, if you 100% believed in your own position you should theoretically have no issue with someone spoiling a game's plot because you didn't yet play it yourself. But my guess is someone spoiling a game's ending pisses you off even though you can still play it. Because you recognize the value of the story outside of the gameplay.
And again, broadcasting something is distinctly different from simply buying it and consuming it as an individual. That's literally why broadcast rights exist. The key difference is in someone making a profit using someone else's works. A radio song playing in the background of a TV show still requires royalties even though the TV show itself is derivative media, even if you only use a clip. There's an entire market for "sound-alike" royalty free stock music specifically to get around this issue.
So yes, this is a more nuanced issue than "lol guys the streamer leads to kids buying cosmetics".
Spoiling a game is different than watching a streamer. If youre actively watching the streamer and get mad because you got spoiled, thats on you. Thats different than if someone walking down the street spoils the game out of nowhere. Again, thats a shitty comparison.
Movies and music you consume by just listening and watching. Youre not playing the game when you watch a streamer. Get this though your skull. ---