One says school prepares us for adult life, which it doesn't at all,
I think we've already established that it teaches a lot of social skills, particularly teaching you how to deal with people that you don't like (which is inevitable), and also get work done in a timely fashion (which is also inevitable).
And neither of these things happen with this free-form learning idea. As stated before, there's no incentive to get used to the idea of completing assignments by certain deadlines, so there's nothing to prepare you for having to learn to do things in a timely fashion, and also no chance to get used to the idea of someone being above you who gives you these assignments.
And as for the social thing, I know you said that there are other ways for kids to be social if school didn't exist, but, you know what? There's still nothing
forcing them to be with people they don't like. It's a thousand times easier to avoid people you don't get along with with no school. And, yeah, it might be the cushier option, but it won't prepare kids for having to work with people they don't particularly like.
This combination of things would leave kids
severely underprepared for life, having no work ethic, probably not educating themselves thoroughly enough, not being used to other people, especially those they dislike...
Just because school may not teach you as many life-skills as it could doesn't mean that it doesn't prepare you for adult life "at all". There are plenty of things it does to prepare you. The system might not be perfect, but kids are still learning a lot more life skills this way than they would be if they stayed home and watched YouTube all day.