You misunderstand the prophecy, or you end up attacking the Bible's account of Jonah the prophet. If a prophecy is conditional, then it won't come to p*** if the conditions change that were necessary for it to come to p***.
Shouldn't God have known the Ninevites would repent? According to your reasoning, either the prophecy "In 40 days God will destroy your city" was false, or God didn't know that the people would repent.
So Jonah's prophecy, that he claimed came from God, was not of God? 40 days later, Nineveh was still standing.
Well, Jonah is indeed dead, so.....
Well, then....
SURPRISE!
What if the DEVIL uses what you've said to UNDERMINE my faith in the OT prophets like Jonah, or what if it causes me to start believing that God
DIDN'T know that the Ninevites would repent? What if what you said results in me becoming an atheist?
Maybe that's why I probably won't ever join a church that has TULIP as its foundation....