Well to be fair, cartoons from that era were designed specifically as part of full theater programs, whose audiences ranged from young children to seniors. The short cartoons were really one of the most essential components of the program in appealing to all demographics, so as such they are incredibly appealing... to just about everyone. Digestible and goofy enough for kids, and smart enough for adults.
There's really very little need for modern cartoons and animation to follow such a broad design methodology, as the theater program no longer exists as it once did. You get purely child-focused cartoons on television that have very little appeal for adults... but they don't need to, they aren't being marketed for them. And likewise you have a more discreet mature audience consuming an entirely different type of animation these days, with mature themes and more radical animation styles and such (think adult anime and adult swim type programing). I see a lot of distress from people over the loss of the widely-appealing animation like the Warner Bros and Disney products from that time period these days, but there's really a much-declined market for that these days, though Pixar seems to still maintain that ideal and are remarkably successful in that (now) niche approach.
Sorry, that was kind of a tangentially related rant, but there you have it.