LLS
I believe most of that change has been on purpose as part of an effort to alter American culture and what it means, core values and ideals, to be an American.
You can see how the comic book medium has changed from decade to decade, especially with the balance between trying to attract an audience/consumers and pushing a political/progressive philosophy.
If you want to understand how things have changed, look the messages the up and coming generations are learning. The messages are very different than they were a decade or two ago and you can document the change via comic books.
Conservatives have a better understanding of how the world works, but we don't know how to teach that, don't know how to sell it and are clueless why the truth we understand is only learned in the school of hard knocks when it could be taught in everywhere.
That is why progressives win now. That's what the progressives figured out and that's why they bought the "means of production" for the social media (who owns ABC/CBS/NBC/NYT/WaPo/AP/ETC.?) and went after controlling the schools (unionized teachers, liberal universities, etc.) and writing the history books.
The societal pendulum has swung very, very far to one side without turning back. It may have been broken or maybe this is another "bubble" like the tech stock bubble, the real estate bubble, the etc. bubble, that is about to get too big and POP! and we're sent back to the School of Hard Knocks to relearn about what is true.