Yeah, all of this. The reason that Replica/Dark Riku works is that Riku was always his own worst enemy; he couldn't allow himself to place sufficient faith in his friends and sought power as a means of securing his own position, particularly in Sora's eyes, as someone to be reckoned with/won back over. He wanted Sora to chase him so badly but he didn't know how to ask for Sora's help in handling the parts of himself that were slowly corrupting him from within without running the risk of seeming weak and vulnerable. This backfired when he walked right into the manipulations of Ansem SoD/Xehanort as a last ditch effort to regain control over his circumstances, and he spent the better part of the series untangling himself from the snarl of his own dark past and the consequences of his actions: so while I thought it was underwhelming to rehash this dynamic once again in KH3, evil!Riku of whatever variety does have some symbolic value as representative of his history and the beliefs he once internalized about himself, and about others. Riku can't ever shirk accountability for what he did, but he can aim to the best version of himself going forward-- the alternate Rikus who appear highlight his growth and his willingness to learn and to change.
Xion, by contrast, walked herself out of the trap set by Xemnas-- in no small part because Riku lent her the necessary perspective to understand her own position and feelings, as she did the same for him-- and derailed the Org's entire plan by helping to move Roxas from a state of guileless stagnation to a point where he was finally ready to respond to the reality of his circumstances. It was, in its own way, cruel in that it meant shattering his idealistic outlook (which Axel's repeated betrayals of his trust had already started to crack) in the most abrupt and forceful manner possible, but the point was to prompt him to start making the difficult choices he'd been avoiding, the things that he didn't even want to look at because it might disrupt the sanctity and safety he had assumed would always persist within their triad. More than being instinctively humble and selfess-- on the contrary, Xion shows a marked capacity for spite ("Do you hate me for taking your friends away from you?") and the burden of her sacrifice weighs upon her, which is why she asks Axel not to hold back-- Xion is clear-sighted and retains an understanding of the bigger picture that allows her to make decisions, even at a cost to herself, if they're for the greater good. She doesn't show Sora's impulsive drive towards heroism but she does share in his ability to take a broader view of the world around her and accept her place in it, rather than trying to inflict her will upon an unchangeable set of conditions: whereas Roxas is motivated to rebel against the unfairness of it all and try to regain what he lost, Xion accepts that loss as part of the cycle of living, which she will remain a part of even after she is "gone," and that her friends will be better off for her readiness to make the difficult decisions they don't want to accept as necessary. So, Xion's primary conflict is not with an aspect of herself that she can't accept, but a matter of coming to a place of understanding her duty to others and the importance of acting in defiance of the rubric of "destiny" as others would preordain it in order to assert her own authority over what is and is not destined for her.
A Dark Xion wouldn't have been Xion at all; there's nothing for her to reclaim because she has no stake in a conflict of identity polarizing good and evil. She struggled with the competing, conflicting demands of a world that didn't seem to have space for her and had to discover her own will within that context: that struggle wouldn't be clarified through having her swing a Keyblade against some proforma "bad guy" version of herself, or having that version of herself swing a Keyblade against Kairi, someone who had nothing at all to do with her journey. They're only "connected" by the raw material of their makeup, but that's like saying Evil Roxas should fight Ven because they have the same face. It wouldn't do anything for either character, at least not without some very explicit groundwork being laid ahead of it to justify the diversion. What Xion needed more than anything in KH3 was for someone to "recognize" her: that was the literal premise of her dilemma, being "forgotten," and the pain of being exited from the circle of existence-- but there was a part of her that persisted in the actions she took to better the world and make a future for everyone possible, and that ought to have been the key to bringing her back, through a reminder that she could never really be forgotten because her will continued to work through those connections she restored through her sacrifice. That makes her a part of those connections, and it just so happens there's a character who is very good at sifting through such chains and identifying the individual links...
The discrepancy between the depictions of the male characters in their return journey and the depictions of women in KH3 is notable. The men are repeatedly empowered through and in the moment of resurrection, with half of the battle being won by them. Ven calls out to Sora actively and basically helps Sora through the process of reviving himself, so it's not that just that it's framed as "cool" because Ven breaks through to save Aqua, it's that Ven is actually a participant in the resolution of his own dilemma. Terra similarly acts to rescue Ven and Aqua before coordinating with Sora to reclaim his own body, and Roxas returns with full cognizance of his own condition and in a moment of crisis where he's shown stepping in to protect his friends from imminent danger.
Meanwhile, Aqua has the crap kicked out of her by Sora before coming back to the constant refrain that she is too "weak," "emotionally damaged," whatever to actually perform heroically, Xion's entire situation is made as confusing and unintelligible as possible to the extent that it's not even clear if she knows what the fuck is going on while she is simultaneously forced into a vulnerable position specifically for the benefit of Roxas's rescue mission, and Namine is essentially killed off (twice?) while she waits around in limbo for the end credits montage.
In its treatment of the femmes, KH3 makes every recent game that actually took its women seriously seem like a fluke. The fact that there are fans practically begging for Nomura to throw them a bone and let Kairi be playable just so they can actually experience the run up to her getting murdered firsthand is indicative of just how low the collective estimation of this game's treatment of women has gotten. Like that is the best possible outcome. Yeah.