Yes i know, that the normal Play Arts Figures never had a Riku. But if i remember correctly, when i was younger, i also wroteI don't think many people are disagreeing with you since, you're assessment (demand = production), is most certainly correct.
Not that this is breaking news, but my contacts who attended NYTF and were told by SE that they "hear fan's demands regarding Terra/Ventus" also asked SE why they don't make smaller figures of more obscure characters. The example he used was Saix (he's not a huge KH-buff) and was told "they don't feel there is a demand for those characters."
So when any character gets made, it's probably a mix of publicity and fan demand. Terra/Ven would probably be in preparation for their continued story (KH3) and because fan's demand. However, it's not exclusively because a few hundred fans sent letters to them. Here's a petition I found that has over 1000 signatures on it asking for specific FFVII Play Arts figures...as far as I can tell, these figures were never made.
That being said, don't give up and keep trying. Better to try than regret what could have been.
This depends on what you view as a "complete set." Just because you view Aqua/Terra/Ven as a complete set doesn't mean SE does. For example, the original Play Arts release of KH2 Sora/KH2 Kairi never had a KH2 Riku come out. They later released KH1 Sora/KH1 Riku, but never released KH1 Kairi. Wouldn't Sora/Riku/Kairi be a "complete set?"
Now with this Play Arts Kai line they have released KH2 Sora Normal/Limit/Final/Halloweentown. Are Valor/Wisdom/Master/Christmastown/Anti a "complete set?" In my opinion, I don't think Aqua HAS to come with Terra and Ventus given she's headlined 2 stories on her own now (0.2 + Secret Episode).
Square Enix already, and asked them to do a Riku and Final Fantasy 9: Zidane and Garnet.
Years later - i have already given up hope, they indeed released Zidane and Garnet!
I grabbed them immediately.
But iam also happy, that other Company will fill the "Gap" that Square can not!
I will wait for this Heartless and Flower Figures, and Nobodys from Diamond Select, and i will buy MORE than one to build
a little Army. The only Missing piece that remains in the End will be Master Xehanort / Xemnas / Ansem to finally complete
an epic Battle Set.
I also understand, why Square says "no need for other Organization Characters"
I also never liked the rest...do not like Demyx or Saix, or that Larxene.
There were only there to be Killed by my Hands - no Important Role for the Story in my Opinion.
But iam happy that they hear the Fans about Terra and Ventus.
Here is the official Email for Square Enix again, that i was told to use for Questions
Everybody who has wishes - try, it only takes a few Minutes of your Life, and raises the Chance to see certain Characters
made as Play Arts Kai.
Of course, i have to say: it would be more easier, if Square Enix would tell us how they "Gather" Informations
or Fans wishes or in General i would like to know how Companies like Diamond Select and Square gather Informations from Fans,
so they know what Fans want and what not.
I only know of Email and Forum so far.