In this chapter of Romans we find Paul explaining why the Gospel of Jesus Christ was opened to the Gentiles. He argues it is because the chosen people of God did not accept the message. So does that mean you and I, Gentiles by definition, are God's plan 'B'? By no means. We have to read more than just Romans 11 of Paul and we need to read all of Scripture to see how this plays out. Yes Jesus did come to redeem Israel first, however, it seems abundantly clear by the end of his ministry the doors of faith had been flung wide open for all who would enter in.
In Acts Peter is corrected by God regarding who and what is holy and acceptable. Paul himself is seen as the Apostle to the Gentiles. It seems there is a warning here in Romans 11 to remember that God is God and we are not. God makes the choices about his is invited and who is not. Another way I am fond of saying this is I am in sales not management. My job, our job, is to communicate the Good News of Jesus Christ with our words, actions and attitudes and leave the rest up to God.
The end of Romans 11 has nice words of benediction to remember God is God and we are not. The reminder that all things are from, through and to God. It is God who is at work in our world and that God has chosen to use you and I, a group of anybodies, to accomplish the great work of redemption. It is God who is leading the way, it is God who is in charge of the whole ball of wax. We are servant/friends of the most high, and God will be God.