My suspension of disbelief is shattered by the cannibalism part. At 17, there's got to be a huge amount of public services under that MC, and when given a choice between selling them off for some paltry sum, or letting them become a profitable member of society, I'd assume they'd choose the latter.
That and birthrates dropping even harder than in most developed countries in our reality, 'cause people who might've had "citizen" children are now lipstick testers, pets or inside "I can't believe it's not chicken" brand of hotdogs.
The slavekeeping part is much more believable, especially if the selection process wasn't vague as all hell and had clear conditions. It could be some form of eugenics, where the gubmint decided that, for example, people with Kansai accents do not represent 'pure' Japanese spirit, and thus are not worthy of being citizens. Or people with behavioral/mental issues, or criminals. But so far, I haven't seen clear ideology behind the whole system, just the "they aren't like us" thing people keep telling themselves and others.
Anyways, I'm interested and will ignore huge holes in the story if the thing itself is entertaining.