God has not rejected his people whom he foreknew. Do you not know what the Scripture says of Elijah, how he appeals to God against Israel?
Romans 11:2 ESV

God has not rejected the physical descendants of Jacob. In Genesis 28:15, God told Jacob that he would not leave him until he had fulfilled his promises. The same principle holds true for all of God’s promises. Over and over, God said that he would temporarily repudiate Israel for rebellion and exile them from the land, but every time he also promised that he would preserve a remnant of Jacob who would repent and return to the land. This is illustrated by the reference to Elijah to whom God said that he had reserved a remnant of the prophets of Israel who had not bowed a knee to Baal.

God's covenants with Israel cannot be ended by Israel's unfaithfulness, because he promises to restore their children for the sake of those same covenants. The “people whom he foreknew” aren’t people of whom he was aware before he created them, but people with whom he has a previously established and continuing relationship. This is the primary meaning of foreknow/proginosko as Paul uses it.