Language complexity

Was talking to my friend Will Pitkin today about my theory on language complexity and streamlining. He suggested to me that in the beginning, about (whenever language began), people used language to name things in the here and now. "Man," "Woman," "Child," "Television." Scratch that last one. But, Will opined, when people started talking about groups or classes, then things got complex. One could not just say, "Man man man man," for the plural. Well, I guess they could, but when you had a large crowd, it'd get tiresome. So, what people needed was a name for the idea of men in general, and some way to differentiate it from the idea of a particular man. Hence markers, or inflections: Man, men. When we had to talk about something that belongs to man, we can simply stack words, (I believe the cargo cult has a name for their religion: "rot bilong cargo," the road that belongs to cargo), or we can add a marker that says, in effect, "of men." From then on, as concepts became more and more abstract, more ways of marking them came into being.
I still think the priests are in there somewhere.

Regularization

I am puzzled by a tendency I see in language. It's kind of two pronged. The first prong is a slide toward making things regular. In English we see this in past tenses of verbs and in plurals of nouns. The past tense of bake was once boke. The plural of shoe was once shoon. Any new verb comes into English with a past tense that ends in -ed (snorf, snorfed), and any new noun comes in with a plural that ends in -s (glun, gluns). The second prong is a tendency to eliminate unnecessary or complex constructions. Hence the loss of any distinction between shall and will, fewer and less, who and whom.
I am puzzled because I ask myself the question, "Why did languages start out complex in the first place?" Lithuanian, which is kind of a linguistic fossil, has 16 different inflections for all those cases. Latin has 5, German has 4, and English mostly 1, or at most 1 1/2.
If we extrapolate backwards, we end up with very complex languages in the beginning. If we posit, as some of my friends do, we have languages that start out simple, evolve into complexity, and then start becoming more streamlined, less complex.
Why should this have happened? The only answer I can think of at the moment is that language came under the control of the priestly classes, who used it as a barrier to the common folk. There is an analog to this in Medieval Europe, when the common folk were discouraged from reading the Bible. Language was the province of the learned and the priests, who stood to profit from impenetrability.
Rebels of various stripes have long known that literacy is a key to the uprising of the masses. Unfortunately for them, when people become literate, they learn to distrust priests of any sect whatsoever, even the cult of St. Marx.