1) YouTube is much more appealing to young children and parents alike than TV, because they are a repository of constant sound and movement and because it's much more manageable to sate if you need to respectively. Toddler cartoons do exist, but they are mostly meant as a "safe option" for parents who think you need to be able to read a little to use YouTube.
2) The audience for several cartoons has shrunk because a piece of the possible audience prefers, say, Let's Plays or FashionTubers instead.
3) With that said, actual children's cartoons are being watched by the intended demographic. The quality isn't nearly Seventies' tier, and the whining about similar styles doesn't really apply to actual children who do not notice or care about "da bean mouth".
4) /co/ is afraid of everything new and hasn't evolved past the first term of the Obama presidency in terms of societal attitudes.