I do think the "decline" of serious superhero cartoons (not just adult ones) is due to a combination of three factors:
1. Networks preferring comedies over action cartoons. We already know how Cartoon Network treated many DC cartoons in the early 2010s. Even something more comedic and toyetic like Justice League Action was treated badly. Big Hero 6: The Series had potential, but Disney bafflingly forced more humor and light-heartedness to the point where it's now just another 11-min. episode comedy. Even DC is producing more comedies like TTGo, DC Superhero Girls, and Harley Quinn, than shows playing it straight. I think Young Justice is their only action animated series.
2. Connection to toys/merchandise. This is not as much of an issue, but it still plagues action cartoons in general. Young Justice, Green Lantern, and Beware the Batman were all killed off directly because of toys. Stretch Armstrong and the Flex Fighters had a toyline, but it didn't really sell because of the baffling decision to keep it exclusive to Walmart, and that might've influenced Netflix and Hasbro not to order more. The third season of Marvel's Spider-Man was blatantly made to sell the Maximum Venom toyline. Not really a superhero series, but Spirit: Riding Free is one of Dreamworks/Netflix's most successful shows despite barely anyone talking about it because little girls LOVE the toys and merch.
3. Already mentioned here, but the popularity of superhero live action films and shows has made cartoons into a lesser priority. Notice how Marvel only has MODOK, Hit-Monkey (?), Moon Girl, Spidey, and What If? in production, and only What If? was highlighted at this year's conference show likely because of its strong connection to the MCU. Also, it's the only guaranteed action cartoon they're making.
That season was greenlit and mostly produced when the show was still on DC Universe, so I doubt the move will influence it much. If they greenlight a season 5, then maybe.