After waiting for generations for Israel to return to God, He gives up on Israel. They had drifted so far from Him, they could no longer be distinguished from those who had never had a relationship with God in the first place. The Assyrian tactic for conquering a nation included moving people from their land and intermarrying them with other groups, so they would no longer have a national identity. This resulted in a mixed race that, in the New Testament, was called the Samaritans.
What a sad thing to reach a place where a person is no longer distinguishable from those who never knew God. You may have dear friends or even children who fit in this category. God was very patient with Israel, but there was a limit. Hopefully you and those you know will never reach that limit!