God’s choosing Israel didn’t mean the people would choose God back. Some in Israel did, but others didn’t. However, God’s promises remained true and steadfast. Each generation and person inherited the promise, but not all lived their inheritance to the fullest.
Again, Paul clearly states that the Jewish people have not stumbled beyond recovery. God’s plan is to bring the natural branches/Jews back in to the vine along with those wild branches/Gentiles/Greeks that were grafted in. The wild branches/Gentiles/Greeks have no cause to boast because God’s grace was given to them while some of their Jewish counterparts didn’t respond to the offer of grace in Christ Jesus.