What does Jeremiah 44:16 show about what God predicted would happen if Israel spared the Canaanites?
Jeremiah 44:16 shows the Israelites responding to Jeremiah's plea to return to Jehovah in the very condition God predicted would result from sparing the Canaanites: they had turned from Jehovah and gone after Canaanite gods, commonly called Baal. This is exactly what God knew would happen.
Les explains that before Israel crossed the Jordan, God explicitly told them not to leave a single Canaanite alive. This was not unfair, because the Canaanites had 400 years to clean up their act but instead degenerated further. God warned that unless Israel cleansed the land, they would become just like them. Israel, however, got soft-hearted, could not put those people to death, and incorporated the Canaanites into their lifestyle. Within a couple generations, Israel turned from Jehovah to the gods of the Canaanites, known as Baal. Jeremiah 44 illustrates this: verse 16 records the Israelites responding to Jeremiah's plea to come back to Jehovah—exactly the outcome God knew would happen if they spared the Canaanite.
As for the word that thou hast spoken unto us in the name of the LORD, we will not hearken unto thee.