"There’s this sort of unspoken rule of community on YouTube, that like we’re all having this same experience together — we’re all watching the same video. That is what makes it a cultural phenomenon is that we all saw the same thing […] so letting people put up two or three different videos of the same thing at the same time kind of fundamentally breaks that, which doesn't feel right."

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It's going to be awkward if you share a youtube link with somebody and what they see is significantly different from what you saw, perhaps even to the point of them replying, "Why on Earth did you send this to me? Are you on crack?"

More importantly, this will almost inevitably lead to content creators being given the ability to not just randomly A/B test versions of a video, but produce different versions of the same video that are shown to users based on their data.

e.g. Shania Twain used to produce different versions of her albums with different instrumentation based on which section of the music store they'd be sold in. There was a Country version for the Country section and a Rock version for the Rock section. She's still bootin' around today, so she could produce different versions of music videos targeting users based on whether Google thinks they like Rock or Country more. This would be relatively harmless, although one might be surprised by the version that appears on a friend's phone.

Musical taste isn't what really divides people these days. What might content creators do if they could show different videos to people based on their political views? This might be good for their business, but it undermines objective reality. People would be shown different "facts" based on their beliefs. This is precisely the opposite of what needs to happen to reduce political polarization and bring people closer together. A common reality is necessary for society to function.