It hasn't shifted so much as expanded. That's what happens when every audience has an outlet that caters to them specifically - a wider range of ideologies gain their foothold. When given the option, apes like us group up and drift towards the extremes over time. The YouTube algorithm works the way it does because, well, it works.
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"The great promise of the Internet was that more information would automatically yield better decisions. The great disappointment is that more information actually yields more possibilities to confirm what you already believed anyway." - Brian Eno
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