Given that public schools can barely teach reading, writing, or math I don't really worry too much about what they are teaching children about much else. It also does not matter what schools anywhere teach if your children do not attend that school.
How do you figure? These people eventually enter the workforce. Maybe they become politicians, and base their political direction on misinformation. Even if they remain in the private sector, someone that is the product of a misguided education will lack the tools necessary to impact society in an informed way, and will have an artificial ceiling on their potential due to lack of knowledge. Education has a massive impact on the world around those who are educated. Considering knowledge to be an isolated, personal thing isn't remotely accurate.
Whenever the evolution vs. creationism debate comes up in the context of a presidential election, some people argue that it doesn't matter whether the POTUS understands biology. But as I wrote, a lack of an understanding of biology isn't the problem; the problem is a disregard for empirical evidence. If a presidential candidate has no regard for empirical evidence in the context of biology, they likely have no regard for empirical evidence in the context of other subjects, including important subjects such as health economics. That's the problem.
So while you're correct that a person can lack an understanding of biology and still be successful, a successful person who has little regard for empirical evidence is a threat to civilization.
I'm afraid that I have to disagree. If a significant number of my neighbors' children are taught that the Earth is 6,000 years old, that blacks deserve scorn as penance for Cain's sin, and that the rapture's arrival is to be expected nigh on any moment now; that is a problem for myself, my children, and everyone in my community. People who believe those things support public policy that is incompatible with the type of place that I want to live.