I do fieldwork on a lamguage that lacks a written form, so I actually know quite a lot of illiterate people, and they are quite capable of complex and rational thought.
In a literate society like ours, we tend to equate language with writing, but they're not at all the same thing. I agree that language is necessary for complex thought, just not writing.
How many truly illiterate people do you know?
I suppose you could say that most 5 year olds are illiterate... are they capable of rational thought, perhaps... are they wellsprings of human knowledge and culture, I personally doubt.