“And if he does have enough time after high school(ie. he gets married a few years later), then he is already serious- so why not give him the necessary courses at that time?”
Have you tried to learn a new language since you were a kid? You presumably learned English from birth (correct me if I’m wrong) and you probably know at least a semi-decent Hebrew, maybe Yiddish, some operational Aramaic from Gemara, maybe some other languages.
The fact is that if I plopped you into a new country and told you to learn the language, it would be much more difficult than it was to learn any of your other languages- not impossible, but definitely harder. Our brains don’t adapt to new languages well after we’ve gotten older- even only past adolescence, which is why I’ll never understand why high schools start us on Spanish in ninth grade. They could do it, but our brains are much more elastic at younger ages, which makes it easier to learn.
Did you know that they’ve tried your idea already? The problem is that many students have quit from the sheer difficulty and magnitude of learning an entire new language, new math skills, new scientific skills at such a late date. Remember, they don’t have that much time to waste learning this stuff- bittul Torah. So they need to grasp all this and immediately, not in stages like how you and I learned it, start on professional training. I know I’d probably crack.