You seem to be ignoring the art styles of the sixties which used limited animation to rebel against the more realistic animated styles used by Disney. Clearly we are just going through that phase again where some animators and people have grown tired of the realistic animation styles and wish for more abstract and less defined art styles.
I think it's more stylistic than anything else. Kim Possible was never meant to be realistic, her figure was barely feminine except for the hair. The more realistic style of Defenders of the Earth or GI-Joe was a choice. The fact that Flash animation is used more now while hand drawn was still used for them may have something more to do with it. It's easier to animate characters like Stephen Universe or My Little Pony with Flash than it is to animated something like Megas XLR.
I don't mind either style, so long as you match the style to the story. I don't like the current version of Scooby Doo in part because of the animation style. It seems so bad after Scooby Doo Mystery Inc.