Here's a mild version of moving away from nice-- Golden Age vs. Silver Age art. I may have picked the distinction up from C.S. Lewis. The examples are my own.

Golden Age art consists of presenting the most pleasing visions possible. After a while, this gets cloying, and some ambiguity gets added.

Early John Carter (ERBurroughs) stories have the most beautiful women and the strongest men. Later in the series, we get focus on lower status people

Superman is pure Golden Age art. Eventually, you get Spiderman.