To be fair, even if your claims about the market are true, books could still be niche. The reason is that books cost $20 and take a few hours to read, so let's conservatively say publishers are making $5/hr of reading time. YouTube is making pennies per hour of video watch time. So if it's true that the revenue of the book publishing industry is approximately equal to that of YouTube, then the average person is spending vastly more time watching YouTube than reading books. In comparison, books are still likely to be incredibly niche.