The problem with a Justice League movie is that Super Friends exists. When people think of JL, they think of how campy and stupid Super Friends was and not the recent more modern JL cartoons.
What people, though? The last Superfriends
cartoons aired in 1986, fully 26 years ago, while the main series ran from 1973-82, over three decades ago. The main target demographic for a superhero movie is going to be adolescents and young adults, maybe 15-25 years old. These are people who weren't even alive when Superfriends
was on the air, but who would've grown up with the DC Animated Universe in the '90s and '00s.