I don't think that's the case. I mean, in some case yes there are companies buying ads when they would have been among the top search results anyway. But assuming that many ad creators are probably savvy enough to know where they rate in organic search results, they're likely not targeting adwords where they'd be high in those organic results.
I.e., competent users of google advertising are, by definition, users that avoid paying for results they would have gotten otherwise.