Even though it does effectively create impunity, this is not a unique phenomenon. Decisions on who to prosecute will always happen, and some form of bias against prosecuting powerful people is to some extent inevitable. So in practice not everyone is equal before the law.  The ICC specifically builds in a way of deferring prosecutions if needed in Article 16 of the Rome Statute. This shows that the ICC accepts the need to postpone cases, possibly indefinitely. The ICC therefore in practice accepts it cannot prosecute in every case where there may be a case to answer.