If the Book of Genesis were never written, would you think creationism was true? Would there even be a thing called "creationism"?
Evidently there would since science posits a creation event. That the Book of Genesis posited the same thing a couple of millenia before Einstein saw the light that a Catholic monk shone on him could be coincidence. Or it could be something else.
Since "Creationism" is a literal interpretation of [the first book of Moses called] Genesis, then your question becomes nonsensical. It would be like saying, If Genesis were never written, would you think Genesis were true?
I know what you're trying to say, but no matter how you approach it, you're going to have a difficult time formulating a coherent set of circumstances, especially given the fact that God spoke directly to moses.
It's one thing trying to conceptualize different lines of events in the past, since these things are beholden to cause and effect, and thus logic and reason...but God, who exists outside these parameters, let alone that he is the creator of said parameters...can not be reasoned to act in any other way than what is already known.
Asking what would have happened if God had not done X is completely incoherent from our point of view.
That's simply my opinion, which you can take or leave...