The assumption that ballads are primal or foundational may be correct, but it has nothing much to stand on.

Without being an expert on either Child or ballad studies, or attempting to be definitive, I'd point out that after Darwin (1859 and 1871), the fashion in Western thinking swung strongly toward evolutionary schemes of cultural development.

These developments always moved from "simple" to "complex."

Thus it was irresistible to assume that the relatively simple ballad must have been ancestral to the complexities of modern "art song."

In broadest outline, this picture must be correct. The most ancient artistic artifacts yet discovered (e.g., beads from 100,000 BC, a bone flute from 40,000) are very simple.

But the cave paintings in France and Spain, which likewise antedate all existing European cultures, are technically more complex than much of what followed for the next 10,000 years.

To make an analogy, Hemingway is noted for his short, choppy sentences, often linked merely by "and." Arguably, then, Hemingway's art is more primitive than, say, Homer's or Henry James's, and to a Martian researcher would "obviously" be far older than either. Of course, there are definitions of complexity other than style, but if we were to conclude that Hemingway is somehow just as complex as Homer or Shakespeare, we're back where we started.

I don't see anything in the ballad's form, or its direct and concise method of sung, rhymed, stanzaic story-telling, to suggest that it *necessarily* arose early in European history.

It's possible. of course, to sing a story without stanzas, or without rhyme, or mostly improvised at each performance, or with non-repetitive tunes, etc., etc. Pre-balladic story-singing of that sort must have arisen not long after singing itself. You don't need the ballad form to sing a story.