it's a bit more fascinating than that. God took all the nations, 70, and set them apart at the tower of Babel. Israel is not one of those nations. God did not single out a nation, He created one from nothing. He repeats a phrase to the prophets over and over, "so that you will know I am YHWH". This is one of those things. Creating a nation from nothing is a testament to who He Is, He who causes things to be. He didn't just grab a nation and say I choose you. He created it.
So we have to say - how did that happen? It began with a single man, then a single son of promise, then went through the younger son, then through Joseph to Egypt. They were the Sons of Israel, who was alive, then the people of Israel who were enslaved - the Hebrews. Then they leave - and the scriptures say God is going to make a distinction with the last plague. Before that, plagues had been falling by geography, and not on Goshen where the people of Israel were. But in the last plague, the Lord says that nothing will happen to the people of Israel. How are they distinguished? By hereditary blood? No! The ones who were the people of Israel were those who were obedient - who took the lamb, ate it, and put the blood on the doorposts. The people of Israel were the ones who did so; the Egyptians were the ones who did not. And when Pharaoh tells them to go, a mixed multitude goes with them. The people who left Egypt - including the mixed multitude - were led by the pillar and the cloud. These exact people were those who St Paul tells the Corinthian gentiles they are their fathers (1 Cor 10). He sees those gentiles as part of the same peopled who came out of Egypt. Ultimately the formation of the people of Israel is culminated at Pentecost, in the covenant of blood. Passover and Pentecost are the two pillars. And the scriptures are crystal clear that gentiles, people not descended from Jacob, were among those who became Israel.
As for grafting in. Yes. Gentiles are grafted in, and become part of Israel -- "All Israel" is the specific term St Paul uses. But in both St Paul's writings and in Acts, it is clear they do NOT become Judaeans or Sons of Israel. St Paul in many ways, but one clear example is not having the gentiles be circumcised, while insisting on it for Judaean (Timothy). Combined with St Paul's insistence that he never abrogated the Law, nor taught those if Israel to not follow the law. Foreigners could not participate in Passover, ONLY sons of Israel (ex 12:43-51). Yet St Paul is clear that the uncircumcised gentile Christians do participate in the Passover in Christ (1 Cor 5:7). The only way to understand these gentiles being of Israel and not under the Law is that they are not Sons of Israel.
At the Council of Jerusalem the Apostles see this the same way. They strictly apply the Torah - the Holiness Code of Leviticus - to the gentiles coming to Christ. In the Torah there are commands said to the sons of Israel, and commands said to all the people dwelling in the land or among the sons of Israel. The Apostles applied the commands to the residents to the gentiles, but not the commands said to the sons of Israel. So the gentile Christians are explicitly part of the people of Israel, but they are also not Israel according to the flesh, or sons of Israel, or Judaeans. Hence the Judaizer's understanding in Acts 15:5 is understood (if they're Israel they must follow the Law) but also rejected.
I never said any promises did not apply to Israel. I said that the promises that apply to Israel are to the Qahal Israel, the congregation of Israel proper, the assembled people of God. These are those whom the covenant at Sinai were made. And that same concept, the qahal, is the ecclesial or assembled people of God in the NT. No promises ended, stopped applying, or changed. They always applied to Israel, and that never was determined by who your mother was. Israel were those faithful, marked by following the Torah and this was shown par excellence in circumcision and participation in the Passover. And, for gentiles who are grafted in, not as Israelites but as gentile Christians, their identity as people is retained and transformed. So in the end, all 70 nations are saved, become grafted on to Israel, become part of Israel, but not sons of Israel -- yet part of the people of God and sons of God, as it says in Hosea.
The promise of Israel is make a new nation to save all the nations, and this is exactly what happens. All Israel saves the whole world. That's what Romans 11 says. "God has consigned all to disobedience, that he may have mercy on all."