If God intended to show Himself to all Mankind through His Creation, why would He not also intend to have His Word be effectual to all Mankind when the revelation of Himself in the Creation was found to be insufficient?
It seems to me that one is assuming that God intended to make His word be affectual to all Mankind. i do not believe that Calvin has yet gotten in to aspects of the Divine Will. At this point, he has only maintained that God has given a (partial) revelation of Himself in the creation, and for a fuller revelation, one must attend to His special revelation, namely the Scriptures.
Calvin is not making assumptions in order to build a theology, he is taking slower steps, and attempting to avoid as many a-priori assumptions as possible. We could for example, question why God would give "general revelation" at all when He knew from all eternity that it would not be sufficient, or even why He did not make it sufficient, thereby eliminating the need for "special revelation".