I don't think there's just _one_ singular "raison d'etre" for school; many different forces have influenced its current-day shape: learning (or rather, the appearance of learning), socializing and friend-making, daycare for working adults (like you mention), etc.

Regarding the socializing and friend making function, if school didn’t exist would that be a problem? Like you have to send your kid to school to be with other kids only because the schools silo away the kids. If there weren’t any schools there would be kids in the neighborhoods. Do kids have to be at desks or in gym class to socialize with each other? I don’t think so?

Socializing (with peer) and allow parents to do that too (work is basically a form of socializing with peer, with benefits) will probably remain far far into the future, as long as humanity exist, whether AGI pans out or not.