We're talking about a conspiracy to hire people for no other reason than to keep them busy so they don't revolt.

No, we're talking about hiring people for this reason, among others.

This is laughably absurd. What do I, as a company owner, gain by adhering to this conspiracy?

You mean a share holder? Nothing. You seem to be laughably under the impression that the company is run for your personal benefit. It isn't.

What I lose is obvious: huge waste on salaries for jobs that accomplish nothing.

You explain why companies hire based upon headcount instead of pure cost. It doesn't make intrinsic sense, but that's how it happens.

This conspiracy does not pass the economics sniff test: it purports a huge artificial externality with no apparent free-riding.

Congratulations, you just assumed away the principal-agent dilemma.