If a prophet speaks in the name of the Lord but the thing does not take place or prove true, it is a word that the Lord has not spoken. The prophet has spoken it presumptuously; do not be frightened by it.--Deut 18:22, NRSVue.
Jonah began to go into the city, going a day’s walk. And he cried out, “Forty days more, and Nineveh shall be overthrown!”--Jonah 3:4, NRSVue.
When God saw [that the people in Ninevah repented], how they turned from their evil ways, God changed his mind about the calamity that [Jonah] had said he would bring upon them, and he did not do it.--Jonah 3:10, NRSVue.
The above Scripture texts show that a prophet, according to the Bible, is not a foreteller of events. It also causes a problem for the Jehovah's Witnesses and their theology regarding what makes a "false" prophet and what makes a "true" prophet.
In Hebrew, the word "prophet" comes from the Hebrew root word nabu which basically means "spokesperson" or "announcer." It is the word used as "prophet" in English translations for Aaron when God tells Moses that he will be Moses' "spokesperson" or "prophet" since Moses cannot speak well. Aaron, of course does NOT tell the future, he merely speaks on behalf of Moses.--See Exodus 7:1.
As the above example with Jonah shows us, a prophet is merely a spokesman for God. And the prophecy or "oracle" (which is a better rendition or translation) is merely a pronouncement or God's message to people. They can come true, but it all depends on the audience and how they respond.
Was Jonah a false prophet since Ninevah repented and was not destroyed? Or was Moses a false prophet since what he said about Jonah was not true since it was God who was merciful and was the one who made Moses' words invalid about what makes a prophet true or false? (And if Moses is false then what does that make the rest of the Bible?)
The problem is not with the Bible, but with a mistaken understanding of the word "prophecy" and what these things are. They are conditional statements, like with Jonah, not what is set in stone. As stated in Jeremiah 18:7-8:
I may declare concerning a nation or a kingdom that I will pluck up and break down and destroy it, but if that nation, concerning which I have spoken, turns from its evil, I will change my mind about the disaster that I intended to bring on it.--NRSVue.
Even the prophecies about the Messiah aren't actually forecasts about an actual individual. There are no direct verses in the Hebrew Bible that say: "There shall arise a Messiah..." or "The Messiah shall do this and that..." In fact, the actual words about the Jewish Messiah don't appear in the Hebrew Bible at all. The reason for the Jewish expectations came likely after the Maccabean Revolt and as a result the Hamoneans crowned one of their own, a Levite, as king, who eventually made a pact with Rome which led to the founding of the Herodians.
Many Jews began studying the Law and Scriptures, and noted that there were repeated promises to David's dynasty to rule forever, claiming that no one else should have been anointed as king. So all texts (via midrash) along this line began to be interpreted in light of hopes that growing oppresions would cease with the coming of an anointed one in the line of David.
But even in this, it was not believed that this was a type of "a magical foretelling of events." These Jews were merely pointing out that the Hasmoneans had taken what was rightfully supposed to be given to someone in the line of Judah. But the Hasmonean dynasty itself was doomed due to its foolish partnership with Rome and the Herodians, and when Rome became the world power and placed Herod in the seat over Judea it had proven too late.
As to the statement: The fact that there would be "false prophets" implies that there would also be true prophets.
That is a logical fallacy.
Just because there are "false cults" does not imply that there would be true cults, does there? Of course not. Just because there are "bad child abusers" does not imply that there would be true and good child abusers.
See? The existence of a "false" or "negative" does not imply or prove the existence or "positive" or "good" of the same or equivalent.