The Jws allow secular schooling because there is no money in it for them to provide schools.
Plus it keeps parents from the preaching work.
So there is no upside for not allowing kids to go to school.
I suspect allowing secular schooling has more to do with legal implications of an outright ban on it - in some countries this may be illegal or otherwise problematic. It would be difficult to justify (though I'm sure they'd manage if they wanted to) such an action from the scriptures, so any government (aka superior authority) would have the final say. That's why I'd expect any action against public schooling to at least start with talks (conventions or CO visits) so that they'd have deniabilty if parents start getting in hot water with the government.
I don't think they care about it actually keeping parents from preaching - the preaching work is more about keeping JWs busy so if they allow homeschooling they'd actually be better off since parents would still have to "get in their time" and they'd have to deal with homeschooling and would thus have even less time to think critically about their belief system.
The upside is (probably) that they'd have a higher retention rate of JW children, and thus better long-term profitibility. That said, they're going to be at least profitable enough, long enough, for the existing GB to live out their lives. That may well have something to do with why the GB doesn't seem to care about youth retention.