The tragedy of Macbeth solely depends on the supernatural, specifically, the witches in the opening scene of the play. Their prophecy led Macbeth and Banquo to believe their fate was set. Their fate may have been something completely different, but their belief in the supernatural led them to make decisions that led them to a different place.
If people of that time had not believed that witches were real, or credible, Macbeth would not have considered their prophecies as truth for even a minute. He would have dismissed their predictions because he would have believed them to be false.
Also, if the sorceress’ first predictions had not become true, perhaps Macbeth would not have gone back to them to receive more guidance. Not having received the second prophecy, he wouldn’t have become overconfident and set up the circumstances for his own death.
If the witches hadn’t told Macbeth their prophecy, I do not believe that he would have ever had the thought to be king. There would have been no story. Nothing else would have stirred in him the desire to be king. There is no plot if Macbeth does not plan to kill Duncan in his pursuit of the throne