You can learn enough survival Spanish to survive very quickly that way, and I highly recommend it. But remember that the people whose accents and grammatical errors we mock here learned their english the same way. If you intend to move beyond survival language skills, you at some point need to do something more rigorous.
It does take years. But when you think about how long it took us to be articulate in English, and to be able to handle subtleties and difficult concepts, that should be no surprise.
It takes both. Neither, alone, is sufficient.
That is absolutely true. Of course, most U.S. children won't ever have that opportunity.
I agree with your concerns about dual language programs. To work well, everyone has to be committed, the teachers need to be enthusiastic and well-trained, and the kids have to CHOOSE to be there.
In Orem, Utah, I saw the results of an *excellent* program that accomplished all these things. Third graders had the fluency of someone who had lived in a foreign country for probably a year. I believe Spanish was the only language they offered at that point, but they may have expanded since then.
But I'm sure this won't be the case MOST places it's tried.