The problem is: Who are the actual descendants of Jacob?Ethnic Israel is assured by God in no uncertain terms that God will preserve them.
They cannot be just the Jews, of whom it is provable that many of them became Jews by conversion. the main example being the entire Kharzarian peoples in the Middle ages.
God does know who the true Israelites are; Amos 9:9. He sent Jesus to save them and now it is the millions of Christians - Jew and Gentile, who are the righteous, faithful People of God.
Thinking that God will redeem and forgive a people group who have consistently and forcefully rejected Jesus for the last nearly 2000 years, is the height of incredulity. They cursed themselves and many Prophesies tell of their virtual demise. Isaiah 22:14, Amos 2:4-5