The way yibum is pulled out of the category of "arayot," the illicit sexual relationships. Where the prohibitions seem to add up, until we realize how narrow a scheme the issue of yibum is, and we don't extend it further beyond the exception. And then the Gemara tries to knock down the rationale it's built up, via the specificity of the language in the biblical verse that teaches yibum. Until that inference is knocked down... All for the sake of handling the co-wives in the best way (prohibited when prohibited, but not beyond the specific prohibition). Not to take this kind of exception lightly!