Honestly, I can see where you're coming from, Esper, but I think that you're falling into the trap that a lot of people do when they experience a sense of nostalgia due to something like a cartoon that they remember watching enthusiastically during their childhood. I'd be willing to say that every generation after advancing into adulthood probably hates the shows geared to the younger generation in the exact same way - My parents would never go anywhere near the living room when my brother and I would watch Inspector Gadget or Captain N, Droids, Transformers, Ninja Turtles - that whole Saturday morning lineup, because my parents believed that they were ridiculous.
We (in general terms) grew up exposed to the age of heavily diversified and serialized cartoons, cartoon spinoffs and an almost unforgiving lack of plot coherence. I recently watched, with some friends from my previous job, the He-Man first season DVD, and although I enjoyed it, I knew when I watched it that it sucked. The plots essentially made no sense, the characters were shallower than a pothole on a newly paved road, and the whole thing was obviously little more than a thinly veiled series of advertisements for action figures, which children (Like myself) would beg their parents for endlessly. I can't begin to imagine how many Masters of the Universe, GI Joe or Ninja Turtle figures I had - even the same character with one or two vague alterations to the body - a new color scheme, a chestplate that rotates to show battle damage, and so on and so forth.
We see these new adaptations of our childhood favorites and hate them because they aren't always one-hundred percent faithful to the cartoons they'rebased on, but then again, the Saturday morning TMNT cartoon wasn't based off of the Eastman/Laird graphic novels, either - the first movie was. Essentially what it all boils down to is: What is popular with children today?
You said something that makes a lot of sense:
The cartoons for kids in the 80's were great. The cartoons for young adults in the 90's were great. The cartoons for adults in the 2000's are great.
That's because they're our generation: The people making the cartoons for adults now are the kids that were watching Garfield and Friends and Ren & Stimpy in their childhoods just like us, who have grown up just like us. So it stands to reason that that would be exactly the way it works.