The philosophical basis of the SJW movement is based around the erasure of history (this is year zero, and next year will be, too) and the "democratization" of the arts (with democratization defined as allowing the ordinary practitioner the same prestige as a master of the craft). This is exemplified in the "Cal-Arts" style of animation: a simplistic style that is easily replicated and doesn't require great expertise to draw (which also makes it commercially appealing, as it is cheap to produce). Many other modern artistic disciplines also distain technical expertise in favor of more pedestrian (and political) styles. Politics (or the implicit philosophical underpinnings of a particular politics) has definitely had a large effect on the kinds of art that have been produced in the last 20 years plus.
This sounds to me like exactly the same thing that people have been saying about modern art styles for over a century - not just the last 20 years. Detractors often say artists in newer styles like Picasso and Pollock were too cheap and/or lacked technical expertise -- "They're too lazy and just not good enough to do _real_ art." Given how frequent the complaint is, it may be true sometimes, but as a broad complaint, it's off-base. It's fine to dislike newer styles of art - everyone has their own tastes - but the changing styles isn't because artists aren't good enough to do the old styles. They're deliberately engaging in the stylization of their times, whether that's Cubism or Expressionism or later style.
Within animation, the thin-line "Cal-Arts" style of animation isn't just cheap to produce -- it also allows more dynamic animation. In a lot of animation with more realistic art, the cost is prohibitive, so you end up with a lot of characters standing stock still as just their lips move or other minor adjustments. The Cal-Arts calls back to earlier thin line styles like Looney Toons when the animation is more the focus, and you can get a lot more dynamic action in a scene. I don't like a lot of modern shows - but then, I thought the animated shows of my youth in the 1980s were even worse.
Regarding RPG art - I generally agree with Ratman,
Certainly. The second image has a "collage" look, where everything looks like it's been cut out of a magazine and pasted onto a background.
But then, a lot of art from older D&D is pretty bad. Or pretty good, depending on the piece. Quality isn't a defining trait of old school.
I agree. Old-school tended to have simple black-and-white line art of highly varying quality. There are some great uses of art, like the POV illustrations of dungeon scenes in Hidden Shrine of Tamoachan, but a lot of books were hit-or-miss in the art - even if the book content was good.