"I can't imagine a single case, anywhere, in which a child was harmed by learning a piece of seemingly arcane knowlege."
Not harmed, no. But we're not talking about a
piece of arcane knowledge, we're talking about many, many pieces of arcane knowledge. At some point the time spent trying (and often failing) to digest and retain all this information that someone else determines you ought to know regardless of its actual relevance to your life, is going to take away from the time you could otherwise be spending to learn things that *are* relevant to your life, and that you will therefore most certainly retain.
There is only so much time in a school day, and only so much time in a life. What if I had the power to enforce that you -- right now -- spend a certain amount of time being exposed to certain history facts that *I* deem important, regardless of your degree of interest in it or aptitude for it, and then demand that you be tested on it? What would that accomplish? Well, you might pass the test, and in another twenty years you might remember a few things. Worthwhile? Alrighty then, I've got a whole curriculum of my own making that you might be interested in.
I think the mistake is in assuming, first, that what is taught in schools are objectively "THE basics" (basic to whom?) and that the ability to retain largely random pieces of information is somehow indicative of one's success in life, and important for other people to know about you.
"Let's say, when you're 40, you suddenly become fascinated in the Civil War and read everything you can about it. Wouldn't any previous background of knowlege in that subject--any at all--be of some help to you? And more importantly, if you were never exposed to the story of the war, would you even be inspired to want to learn about it?"
I've learned and become interested in plenty of things in my old age that I had NO preparation for or exposure to in school. Life doesn't end with school. And the thing is, school can't prepare us for everything we might possibly be interested in somewhere down the line. Say your life-long interests included fine art, metal working, community service, and the Japanese language, and that you have zero interest in basket-weaving, horticulture, quantum physics, and anatomy. Yet your school experience focused on the latter. What good would that be doing you in the present to have been forced to learn those things in the past? And what good would it do for you to be told that these things were Extremely Important and that if you did not do well on the Tests it foreboded bad things for your life?
Again, the issues are whether the basics being taught are truly basic and necessary to all, whether textbooks are the best -- or even good -- sources for that information, whether students will retain a significant amount of information when they are not interested in it to begin with and when the subject is not taught in a meaningful context, and whether standardized tests really tell us anything at all about the true development and abilities and potential of a person.
As far as I can see, the answer to all is a resounding "NO".