The reason for this isn't "cowardice", though. I've read a lot of things by practicing people in the field on reasons for which they don't think human challenge

The reason for this isn't "cowardice", though. I've read a lot of things by practicing people in the field on reasons for which they don't think human challenge trials produce reliable enough data. (Basically: You don't get a representative sample, and you get an unrepresentative sample in ways that are very likely to result in you getting bad data.) There's other reasons to distrust them, and I don't think your assumption that "an absolute minimum of risk" is really on the table is even defensible. And on the whole I think we probably should be using them anyway, with caveats -- but your argumentation here is bad.