Unless I'm wrong, which is entirely possible, neither one of us can show such a "study", since they are merely positions we're holding. Mine seems far more common sensical, since yours seems to be companies hire janitors simply to hire janitors. And because some don't, ipso facto, that supposedly proves your point. Hardly.
Point now is you can't prove that a janitor is not hired, based solely on their negation to the bottom line of that company. For some reason, that just makes sense to you. As is the salseman who should be taking out their own trash, to apparently help that company's bottom line