Timeline-wise, Riku literally just became a Master going into KH3. Riku constantly fighting with the darkness at this point would've been redundant, because it's just been happening for so long now that you just want to move on. KH2 was him being ashamed, and if he didn't face it in DDD, he could've very well cost the Guardians of Light their victory.
I honestly doubt many people were expecting Riku to still be struggling with the darkness after DDD. But as a newly named Keyblade Master and someone who walked the "Road to Dawn," Riku really should have (in my opinion) been shown imparting wisdom and knowledge to those who were going through a similar struggles to his own. Such as giving Terra a new perspective on the darkness like he did for Mickey, or... helping Aqua overcome her fear of it (like he himself did back in Reverse Rebirth). It could've provided some important and
profound moments for the character, while accentuating his growth and prior struggles. Also, let him actually
accomplish something important (because, similar to Aqua, Riku didn't resolve the conflicts he was set up to resolve in this game).
You don't need to back track on his development, or even give him that much more screentime in order for Riku to have a stronger and more fulfilling presence.
Instead, he was basically the leader. Aqua certainly wasn't, Mickey wasn't either, and Sora absolutely wasn't the leader. Riku doesn't need the focus because he's faced what he needed to face. He found the strength to protect what matters.
Hm? At what point did Riku direct, organize, encourage, or strategize for the GoL (as a unit) in such a notable way? Certainly not enough to be considered the
leader, from what I recall.
The GoL pretty much went into the fight without an actual plan or strategy, and different people were giving orders and suggestions at different points. No one, aside from maybe Yen Sid, was shown consistently being framed as the organizer of the group. Or am I missing something?
And think about it. Everyone else in the Keyblade Graveyard gave up. He was the only one that went down fighting.
That's.. not a moment people appreciated (or one that we needed to be forced to watch again), lol.
There's great Riku moments in KH3, they're just not that often because now that Riku is able to stand on his own feet, Sora needs to focus more on the people who can't.
I mean... KH3 tried that, and Sora's focus as a character was all over the damn place.
First he's trying to regain his lost powers, then he's trying to find a way to bring Roxas back, then he's told to drop that and start looking for Terra, then he's suddenly worried more about Aqua (despite Riku and Mickey already taking care of that), only to find out that he had the PoW all along, and just needed to "call out to it" (because I guess he never bothered to check if he could earlier). It's incredibly disjointed. Almost like the writers want him to focus on
everything, and it just doesn't flow well at all. He's better off having a single, more streamlined goal (like he did back in KH1).