The Bible has history, poetry, wisdom, parables and law. It was never intended to be taken literally. If that were so, it would not have stood the test of time. Not all humans are simpletons.
Ancient people understood as well as we do that nobody could survive three days in the belly of a fish.
Christians, however, maintained that with God anything was possible, and therefore Jonah could have survived being swallowed by the fish.
But they also understood Jonah allegorically because Jesus did, as a foreshadowing of his own death and resurrection.
And ultimately, Jonah's historicity shouldn't be treated as a stumbling block; if that's the only thing standing between the seeker and faith, let them believe first and unravel the mystery later.