I'll admit I've only skimmed some of the work you're citing, but has anyone focused in more on the "pure preaching" and "proper administration" aspects? One argument I can see is that the gospel is not preached purely if the law is not also preached purely, and calling something not sin that is sin dilutes the purity of the law.And that's exactly the place I want to begin, because David put his finger on one of the key complaints about preaching the gospel in its purity, the one that is most often framed in terms of "Gospel Reductionism." It begins with the notion that the gospel requires the law -- that the dialectical relationship between law and gospel is necessary for the existence of the gospel. The second move in the chain is, given the scriptural understanding that the gospel has an essence -- that as Paul says there is one gospel and only one gospel and it is thus-and-such -- that therefore the law has an essence. That therefore the purity of one depends on the purity of the other, and any dilution of one has a direct effect on the other. And so if we water down the law, we cannot be doing justice to the essence of the gospel.
(I might note that the argument rarely runs the other direction -- nobody shouts so loudly when we water down the gospel and deliver full-strength law. They just blame it on the nature of Christianity as a judgmental and condemnatory religion. It's expected.)