"If the language designers find it useful to occasionally use type-parametric functions, why won't they recognize that other people might also find it useful, too, and for other functions?" doesn't strike me as a demand for purity in any sense. Consistency, maybe; recognition that the designers are giving themselves special treatment, sure. What's pure about the desired state of affairs, or impure about the present?
Honestly, sometimes it seems as if someone who wants to defend Go against a criticism immediately claims that the critic is just obsessed with purity. Why else would you criticize Go?