Or "taking" fewer customers than naive interpretation of sales would suggest.
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It is not valid to assume that people who buy Pathfinder are not also buying 4e products, or vice versa.
Kinda yes, kinda no.
You are certainly correct about the fact that buying Pathfinder does not preclude buying D+D products. However, it's also true that gamer's funds are not infinite. A customer who buys a Pathfinder book is less likely to buy another D+D book. It's probably more accurate to say that Pathfinder is taking sales, not customers.