The principle of jus cogens holds that there are some rules of international law that are so fundamental that they may never be abrogated by a treaty. For example, it would be improper (not to mention somewhat illogical) to make a treaty saying that countries do not have to abide by the agreements they have made in treaties. One fundamental law (jus cogens) is that countries must do what they have promised to do.
To argue against this idea, we need to problematize the idea of fundamental laws. The idea that there are fundamental, overriding laws assumes that all (or almost all) countries will be able to agree on what those laws are. However, as we can see in cases of things like human rights, not all countries do agree on things that seem fundamental to many others. For this reason, it is very difficult to point to any fundamental principles of international that could possibly be so widely accepted as to have the force of law. Therefore, we should not allow any country or any jurist to overturn a treaty based on their feeling that it violates some peremptory principle.