A litle too extreme, streamers just share content is up to the viewers to decide how much to watch, plus I doubt most people who really want to play the game would watch a stream first.

Why would it be any different? 

It's their game, they can do whatever the fuck they want.

If I were to release a game I'd partner with certain big Youtubers and allow them to post videos about it, up to a certain point.

I would block lets players who upload entire playthroughs on youtube, that shit cost sales.

It should be general policy actually, your game gets exposed and have people talk about it, but also cuts off those who wouldn't have bought actually to buy it.

It's not like you have to watch the videos. Just play the game and ignore them.

Is there any proof at all that playthroughs lower sales of games?

Certainly there are people who bragged about having watched TLOU on Youtube, I also personally know people who do it.

How do you know they would have bought the game if they hadn't seen it  on YouTube?

My bro watched RE7 and now doesnt want to buy it.

If a person gets far into a game, more power to that person. Since we have the technology to showcase a game, common sense dictates to avoid spoilers. And if you happen to come across them, oh well. I have more important things to do than scream over spoilers because I have a life.

But would he have bought it beforehand?