Who is shotacon even for? Do you guys want to be little boys who get molested by a woman in her late 20s? Is this for Women who are into little boys?
For shotacons, naturally.
But to be less smartassed about this...
Speaking of shotacon in general: there appears to be a correlation between shota and boys love worthy of observation; in contrast with
bara (marketed primarily towards gay men), characters in boys love (marketed primarily towards girls and women) are more "ikemen" and/or somewhat androgynous in appearance. Shota, obviously, tends more towards androgyny, seemingly coinciding with the aforementioned boys love tendency (especially since gay shotacon is common enough that "straight shota" is frequently specified). Fundamentally, they (often being women) find these characters "cool" and/or "cute", whether they sexualize them or not.
Speaking on onee/shota, since this is an onee/shota manga and it's what you're actually asking about: it's the fetish complex of female domination, size difference, and age play taken to its logical conclusion. Rather than the male
acting like a baby, they-- at minimum-- have a body that is able to be likened to that of a child, and in some cases
are a child. Rather than being persuaded into submitting to the woman, or the woman using techniques or substances to render him unable to resist her, the very stature of the male usually renders them
incapable of resistance. Where the shota ends up being the "dominant", the appeal appears to be about the irony of such circumstance. The primary audience of onee/shota appears to be men, and the men do not appear to have sexual attraction to the shota as much as they seek to identify with him or otherwise see him as a kind of "tool" for the "onee-san" to act upon.
And, yes, there are people who want to be little boys being molested by a woman in their late 20s. And there are actual little boys who at least believe they want to be molested by a woman in their late 20s.
Generally speaking, when you make the mistake of wanting to examine sexual fetishes among non-criminals, you don't want to assume that there's a 1:1 correlation between what they view and what they actually desire, and you
especially don't want to do that in regards to cartoons (which are extremely and multilaterally unmoored from reality).