I'll put in for the "logical win" based on the explicit premise of the show --- Sia+Nerine+Kaede for the polygamy win .... that closure might have worked.
However, its not that the Asa+Rin was a "innovative surprise ending" since the plot started telegraphing it in episode 1 ... even having Nerine point it out multiple times that Asa loved Rin (bright girl, Nerine).
Unfortunately, when they went for the Asa closure --- the transition from Kaede's arc left Asa looking like some fiend who not only trampled Kaede in her rush to Rin but showed absolutely no interest in her close friend's mental condition. Totally unlike the Asa they had spent nearly 18 episodes developing. Then they compounded this writing disaster with some of the lamest motivation during Asa's arc that I've ever seen put to film ("I'm going to die because I don't want to see my mother cry." -- I guess its okay for Ama to cry after Asa dies???).
Even if one is fond of Asa, the dialog and plot was total WTF? Asa, all the way up to Kaede's outburst at the doorway and Asa fainting -- made sense. Her scripted behavior after that made no sense.
The script of Rin and Asa's abandonment of Kaede to her nervous breakdown was so totally out of character and cruel, it completely destroyed the respect carefully developed over the first part of the series.
Perhaps they thought they were writing some Shakepearean riff where you're supposed to loathe the protagonist and his queen who turned from good to evil at the end....
Probably the most obnoxious part wasn't even the tragedy, negligence, and deceit --- that can be just good melodrama ... it was the last 15 minutes of the series where they tried to pull off the lamest "harem reset" I've ever seen. After all the pain and growth the characters went through .... they're sent back to square one "chasing Rin". I suppose that could be some kind of just desserts --- Asa forever having to watch her rearview mirror because the other girls remain in pursuit.
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But..
I'm not an Asa-hater --- Here's my quick synopsis of the way I would have treated the last 5 or 6 episodes with an Asa ending:
No change up to Kaede's outburst in the hallway with Rin and Asa (frankly, the drama of Kaede's slow breakdown was dramatically brilliant).
Afterwards, have Asa actually ask about Kaede in the hospital. How's she doing, Rin? Maybe have Asa say Rin needs to address the situation with Kaede before they can move forward. (that'd be the Asa I know).
Have Rin explain to Kaede that he's decided on Asa, that Kaede needs to move on. He'll help but she needs to move on. This doesn't have to go well.... in fact, dramatically I'd expect Kaede to attempt suicide. Perhaps have her at the bridge... and *thats* where Asa arrives for their pivotal scene together ---- extend the conversation, have them cry it out together. The situation is what it is but Asa makes it clear they will *not* abandon Kaede.
And then have Kaede *move on* ... building a home with Primula, starting to notice other guys.
For the Asa arc, alter the "won't use magic" because she's afraid of the power, afraid she can't control it like her mother couldn't. She's mistakenly afraid her mother might reject her if she uses it because her mother rejected magic so strongly. Now the "won't use magic" makes some sense.
At the end, have the girls move the fsck on.... Nerine with her new social life and activities and hooking Sia along, Kaede and Primula head out shopping *for themselves* ... if we must have a "game still on", they can hurl some light comedic remarks about appearing so stellar that Asa better stay alert or Rin might get a roving eye.
If they'd done that I'd have marked the series a 10, Asa ending and all.
p.s. .... I'm not sure why Sia and Kaede kissing bothers ya'll so much. I don't think it was a complete accident so much as a bit opportunistic on Sia's part. Sia is a very adventurous little puppy and I wouldn't put it past her to be bi-curious. Kaede looks pretty astonished though
A kiss does not lesbians make.. not does it not make them lesbians.... it was a kiss.. not a sex scene.