Tangentially, last night I half-listened to an interesting YouTube video that used the history of Robin Hood movies to describe this trend.
https://www.youtube.com/watch?v=428ZxrW6jhk
It's a slowly escalating 45-minute rant, but I think that this quote from toward the end summarizes it fairly well: "I'm admittedly a little tired of seeing heroes always surrounded by worlds of gray, because, if they're there long enough, they start to feel kind of gray, too."
I haven't ever seen the Errol Flynn version of Robin Hood, but I suppose my equivalent is that, as far as I'm concerned, Batman peaked with the TV show in the 1960s. It wasn't just colorful, it was legitimately fun. To the point where even the bad episodes were good. The Tim Burton movies were also a bit like that. They were visually dark, sure, but that was Tim Burton's aesthetics, and it was a package deal that came together with at least a few glimmerings of that same twisted sense of humor that got him fired from Disney for making Frankenweenie.
Since then, though? It's a bunch of increasingly sad movies by apparently sad people whose creative drive seems to primarily come from the desire to demonstrate to themselves and everyone else that they are Grown Ups, and who are too busy Taking Their Jobs Seriously to have any fun at work. And so they're working so hard that, even though what they're producing is technically classified as entertainment, the end result is so joyless that watching it ends up feeling, at least to me, like work.