I like how it seems to contradict everything we think (correctly or not) women are into.
Shotas are younger, women tend to prefer older men, they are smaller, weaker, more submissive and innocent (assumedly, I do love the occasional shotadom), women tend to prefer the opposite to all these.
/ss/ with very young shotas (like younger than 8) with small pps (4cm or less) tend to be the best kind of /ss/ out there, and surprisingly rare as most artists are resistant to draw ochinchin that small. It's not aesthetically unappealing.
The claim is that oneechan wouldn't be happy with it, but keep in mind that yuri exists and /ll/ exists as a subgenre in that, this proves that the oneechan could have a wonderful experience with even less, yet nobody expects them to enjoy themselves with someone who's got a bit more to offer?
Assuming the oneechan isn't the giving type who simply enjoys being the source of stimulation for her partner (commonly seen in yuri). If she either teaches him to use what he's got or if he's already got the skillset he needs to make the most of his opportunity (such as in the equally rare shotadom stuff) she can still enjoy the experience quite a bit. Even without that there's the psychological factor to female sexuality that allows them to draw the most enjoyment from the experience from her mental pleasure rather than from the physical part of it.
The concept of conquering a giantess or being the object of her desires is definitely an element of it, it sort of folds into the list at the top, smol-d shota exaggerates this part since you are not only conquering her or drawing her to you, you are doing it with substandard equipment for the job as well.
The mommydom element is there too, I guess, she's not just a partner, she's a teacher and caregiver as well. There's a common focus on breasts in /ss/ for a reason, as well the sex itself could be seen as a return to the womb, in fact /ss/ often appeals to the idea of being literally born again, but doing better this time, the self-insert element is real.
In shotadom the appeal is still there, but it takes a different form. I guess there's the loss of innocence / coming of age appeal in mommydom, but in shotadom it's the idea of a false assumption of innocence being taken advantage of, also of a status-inferior male dominating a status-superior female partner who nobody expected to overcome.
This is a common trait between loli and shota, the sexualization of the defiance of the cultural expectations, it's deviant everywhere, but in japanese society it deviates on multiple levels.
I think it appeals to the extremes in society, either very successful and the very unsuccessful, both have stresses that would lead them to this point. For one it's empowering escapism, for the other it's disempowering humiliation.
Not much else to say, though as a last thought there's the appeal of how /ss/ challenges what you perceive to be the ideal male. As stated before shotas, especially young ones with small dicks, are the exact opposite of the ideal man in many ways. Yet /ss/ treats these as positives.
For the ideal man acting childlike is something shameful, especially today, but /ss/ frequently affirms the positive ways that behaving like a boy could be framed, very often it's the boyish qualities of the shota's personality that makes the oneechan desire him, even on a physical level.
The need to be nurtured, provided for or protected, serves as the prelude to sex, this is the role of the ideal man and it's being taken up by the woman, the specifics of the situation allow the role reversal to become sexual.
I got into /ss/ by the south park episode that covered it, "miss teacher bangs a boy".
In reality the teachers were pedophiles only in the legal sense of the word, as while all the high school boys they had relations with were minors they were also all past puberty and tended towards being the older looking members of their classes.
In the episode, however, the teacher was teaching kindergarten or maybe running a daycare, either way she was banging a character who was a toddler.
Her attraction to him was precisely the things that would turn most women off from him aside from his age, that episode resonated with a lot of adult men who either retained some of their younger traits, or the exact opposite who would want to be loved as that kind of man.
Really this genre is about what adult men want from adult women, and it's not ageplay, it's acceptance and love for what society sees as their failings, and it is the assistance to grow out of these things.
Yes, the appeal of /ss/ might just be the exact same thing that gave us the manic pixie dream girls of the 90s and 00s. If only some women would come and take me in my damaged state, and give me a reason to better myself, then I could become what she wants me to be.
The oneechans take a shota and patiently raise him to manhood through sex. MPDG's did the same with manchildren but through means other than sex.
Kind of like those guys who fantasize about some apocalypse situation where they could prove they were a chad in disguise all along. As loner dropouts from the system they would be better equipped to live apart from it and survive, they guess (wrongly).