I see that you look at minors debuting through a business lens but I think the main issue is sexualizing them because in this world some people find high schoolers hot. This was the case for NewJeans (no I don't wanna hear you explain how that song is about baking bffr it's not) Young Posse, Yeri from Red Velvet, TWICE, and HyunA (back when she was a Wonder Girls member for some reason but in her career in general)
I have no issue with groups like NewJeans as long as they're being treated fairly and singing songs for their age group because I don't wanna hear a barely high schooler sing about how she wants someone to come take a lookie and taste her cookie
the sexualization isn't unique to kpop but it does happen more often and if you were / are a prospective kpop trainee then it's up to you to understand such things and (not be ok with it) but rather understands it happens and deal with it accordingly...it's not the minors fault for being sexualised that's on the people who are sexualising them...(and sometimes on the company for failing to protect them)
but that's you though...if you didn't like Cookie then so be it don't listen to it (I don't like it either lol)
if you think the lyrics are inappropriate then you have that right to call them out / not listen to the song / cancel the group etc etc
but there are other people with different views and don't think it's inappropriate - they have the right to their views as well
in terms of the business aspect of things...well literally all the people you've mentioned in your post (save for say young posse) are somewhat successful - I am not a success stan - you'll rarely see me even commenting in a "success" thread but it doesn't mean it doesn't matter...
for us as fans we want to see our favs happy and successful...and if they don't "succeed" during the first contract period it's very rare that they'll succeed after that
which therefore brings me back to my first point - kpop is a business - our idols may produce content and songs and music and whatever but that's their job and as part of one's job is to earn money!!!! and they don't get a salary - their entire career is a commission based gig - you sell more chart higher get more cfs tour more sell more merch make a name for yourself and you earn more money
if you don't - well like the people I mentioned in my above post earlier - for every jisoo/nayeon/irene there are probably thousands of unknown or unsuccessful idols