I'd say that relative to 4E it actually failed at the latter. 4E's DDI let you create monsters a lot faster than 5E does by any method, and 4E actually had rules for creating monsters, unlike 5E 2024 which now has none lol.
(It's also not much faster at creating characters than 3E/4E/PF - it is faster, but not vastly so - what's much faster is levelling them up though, far fewer choices and importantly - basically no required "planning" unless you're multiclassing.)
Yeah, character creation in 5e is still very complex. I've been playing Ironsworn with Hussar and his gaming group (which I guess is also "my" gaming group now? It's been a year already, sheesh!), and creating characters there should never take more than about five minutes even for an absolute beginner. Pick three assets that don't have prerequisites (so, 3 things from a pool of like 40ish?), assign your score array (+3, +2, +2, +1, 0), come up with your character's core quest or quests (formally, "Vows"), done. The world-building questions bake in starting Vow possibilities.
What's funny is I'm not sure that "younger audiences" really even want products directed specifically at them. As a young teen I remember nothing irritated me quite as much as well-meaning attempts to make something for my age group specifically, which always ended up feeling like it was for "kids". You also don't want products directed at grogs of course, to be fair.
I think capturing the zeitgeist is something pretty distinct from this, and that TSR actually did a better job of that than WotC has really ever achieved, when they managed to drop Dark Sun and Planescape within three-four years of each other, both of which spoke to "youths" of that era in a way that nothing WotC has ever made has (the closest being the general, vague "dungeonpunk" vibe of 3E, but they never capitalized on that by, say, making a dungeonpunk setting, because Eberron, despite being cool, sure ain't it).
Yeah, if you're going to try to tap into that, absolutely do not ever make stuff that is "for kids."
Avatar: the Last Airbender is a good example of what you
can do to market to younger audiences. Build things that are deeply authentic, that take the time and show the work. Don't shy away from Adult Themes, by which I
emphatically do not mean gore nor sexytimes, but rather adult-scale tragedies and the darkness that can really exist in our world. Build up a setting that has an interesting premise, and a great deal of potential for something engaging: discovery, or change, or meaningful conflict, or personal expression. Preferably, as many of those things as you can squeeze into it. To continue the example, the world of
Avatar feels like it's full of so many things to discover, particularly because of the hybrid animals and the deep world-building but the story's need to move
quickly through that world-building. It's a world definitely in need of change for a variety of reasons (even if I don't like the specific changes that
Korra made to it), and it absolutely has numerous meaningful conflicts going on, not all of which get solved over the course of Aang's story. And "personal expression", my
goodness yes! Bending as martial arts that create (constrained) magical effects is brilliant, since there are far, far more martial art traditions than just those shown in AtLA.
That's the kind of thing you need to do, and I think it's why Eberron didn't quite make the mark. It was close, it had quite a bit of potential, but didn't
quite land.
I will say, I don't think a setting as aggressively nihilistic and dystopian as
Dark Sun is likely to be a candidate for this. You could work with dystopian or nihilistic concepts, but there would need to be counterbalancing things, like maybe the setting is kinda
zeerust-y, full of sincere humor and heart to counterbalance the dark elements; or maybe it's a much darker take on 4e's "Points of Light" concept, where society isn't just teetering on the brink, it's functionally collapsed outside of a tiny handful of bastions barely keeping out the night from their own walls.
But perhaps that's just my own biases talking. I've grown incredibly weary of the over-the-top grimdark stuff, whether it be "grimderp" where the setting is constructed to be SO overwhelmingly bleak and horrible that it crosses the line twice and becomes distractingly goofy, or "shitdark" where the setting is merely overwhelmingly bleak without crossing the line twice and thus just feels...awful for seemingly no reason other than to revel in awful people being awful to one another for as long as there are other people to be awful to. That's why I prefer what I call "chiaroscuro" fantasy; the dark is
there, and
real, and
dangerous....but the light is also real and knows how to be dangerous in its own way.