Certainly, theirs a potential for that to happen. But is it a necessity for almost everyone to know magic, in such a world? Is it really unbelievable to have a large number of people who could potentially cast spells, but don't actually know any spells?
Let's try an experiment and replace a couple phrases in that statement.
"I find it hard to believe that, in a society of humans where learning to read and write was as easy as copying a few symbols out of a book and memorizing the sounds they represent, there would be any significant fraction of the population who was illiterate."
Ignoring arguments about the relative difficulties of learning one system of writing or another, for a significant portion of human history, reading and writing has been that easy, and yet illiteracy was the norm. Why?
- Partially because children who could learn to read could also do work, and that's what most families made children do instead of learn to read.
- Partially because, unless someone offers to teach people to read and write for free or the government pays for it, it's expensive and only affordable for the rich.
- Partially because books are so expensive to create for most of human history that they are luxury items and pretty rare, so there's not much demand to learn to read and write.
- Partially because some segments of the population weren't allowed to learn to read and write, and in some times and places, a small caste of literate people tried to keep literacy to themselves, to retain control.
This is relevant to learning spells, as well, at least as imagined in the D&D universe. Learning magic requires learning to read. In fact, it may require learning a specific language or writing system that is not in common use, much as, at several moments in history, there were two writing systems or two languages in use, one profane, one sacred. Egyptian hieroglyphs get their name from the fact that they were used by priests. And, as it turns out, they were associated with magical knowledge and spells as well.
The first thing we need to remember, then, is that the potential to learn something does not mean there is also an opportunity to learn something. The second thing... is what I'll cover in a post tomorrow.