I 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.
https://www.ipetitions.com/petition/ffviiiplayarts/
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).
Yes i know, that the normal Play Arts Figures never had a Riku. But if i remember correctly, when i was younger, i also wrote
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
enquiry@square-enix.com
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.