Nowadays, kids that age can’t handle a full day of learning, and a secular education, aside from having its own value, is far better than the other ways the boys would otherwisw spend their time.
Sure, it would be great if kids could keep an intensive, 12 hour a day learning schedule. But they can’t.
Who said anything about a 12 hour learning program? If the kids are already keeping an 8 hour learning program — split between Chumash, Gemorah, Meforshim, science, English literature, and history — they can handle an 8 hour learning program of Chumash, Nach, Gemorah, ??”?, and additional Meforshim.
Same hours, no secular studies, more Limudei Torah. And it consumes the same amount of time as a program with secular studies.