Your comment is in the surface a rebuttal of the proposal but it really does not address international criminal law (ICL). ICL is also the result of a treaty between sovereign States that agree to criminalize certain behaviors by individuals and, in the case of the International Criminal Court, agree than an international tribunal will prosecute those crimes when national authorities were unable or unwilling to do so. This complementarity aspect has led to the widespread national criminalization of international crimes and thousands of prosecutions against war criminals or perpetrators of crimes against humanity. Arguably, it has shown effective.

But ICL goes beyond the Rome Statute and includes e.g. treaties on terrorism or piracy that include "prosecute or extradite" clauses.