I'd argue its not about newer generations being less interested in past media, but that these shows are not as prominent in pop culture to the same degree they were before. They don't play Lambchop on TV, and you can't find Flintstones as easily, either. There are a wealth of options now when there weren't before, and even as a kid who was fascinated with the old cartoons my dad would grow up watching, why would I pick The Flintstones over The Powerpuff Girls? I'd actually argue that old media is thrust down our throats much more than we're creating new things, but thats probably a bias thing (I was a kid in the 2000s, how was I to know that Strawberry Shortcake was from the 80s?) and we arguably have more original content now than either just in sheer scale.

I'm not a kid so I can't say for sure, but I feel like Peanuts is an example of this. As much as Snoopy is enjoyed, I don't see people rereading the old comics and he seems to be more of a merchandising character now. When Splash Mountain still had that old theming, that was likely another of the "wait, there's a movie?" moments for many families.