I agree with you here, but I disagree with the idea that the body of Christ is now Israel.
We understand Paul's final point based on what he has already said about Israel in Romans 9 - 11. He does not discuss the unification of Jews and Gentiles in those chapters. He is attempting to explain why God's promise to his kinsmen hasn't failed because, for the most part, many of them are not coming to believe.
The framework of Paul's arguments, starting in chapter 6, consists of a series of rhetorical questions. Therefore, to understand the Olive Tree metaphor, a Bible student must recognize that it aims to address the question: "I say then, they did not stumble so as to fall, did they?" If our interpretation of the Olive Tree metaphor does not provide an answer to that question, we can conclude that our interpretation is incorrect.
How does the Olive Tree metaphor speak to a possible fall of Israel? He continues,
Romans 11:15-16 For if their rejection is the reconciliation of the world, what will their acceptance be but life from the dead? If the first piece of dough is holy, the lump is also; and if the root is holy, the branches are too.
The Apostle uses an analogy to compare the nation of Israel to a piece of dough. This analogy relies on understanding leavening, which is a quick and transformative process that rapidly affects the entire lump of dough. Therefore, if a piece of dough is free of leavening, then one can conclude that the rest of the lump is also free of it. Similarly, if the first piece is considered holy, it follows that the entire lump is also holy. The part stands in for the whole.
He notes that presently, there remains only a remnant of Israel that is coming to faith. So he asks whether a remnant of believing Jews is enough. He argues that even though a partial hardening is taking place and only a remnant are being saved, since the partial is holy so is the entire nation. The part stands in for the whole.
The same analogy holds true for a holy tree. If the root of the tree is holy, so are the branches, and since Jacob is holy so are his children.
It is important to understand that the branches in this context do not symbolize individual people, but rather represent generations of people. The notion that an individual Jew would be severed from the tree to make way for an individual Gentile to be grafted onto the root is inconsistent with the principles of righteousness. Such an implication would contradict the nature of God, who is fundamentally just and would not engage in such actions.
The key to our understanding is Paul's reference to making some of his countrymen jealous. This idea comes from the book of Deuteronomy.
Deuteronomy 32:21 ‘They have made Me jealous with what is not God; They have provoked Me to anger with their idols. So I will make them jealous with those who are not a people; I will provoke them to anger with a foolish nation . . ."
The branches represent generations of Israel as Moses describes here:
Deuteronomy 29:14-15 Now not with you alone am I making this covenant and this oath, but both with those who stand here with us today in the presence of the Lord our God and with those who are not with us here today.
God made a generational covenant with Israel. The covenant comes with blessing and cursing. The generation that keeps God's commandments will experience blessing, and the generation that worships idols and other gods will suffer cursing. Some generations were cut off from the root, while other generations remained.
The same is true of Gentiles. After Jesus, some generations of Gentiles were grafted onto the root. Someday, according to Paul, the Gentiles will be cut off. This will take place in the future.
While Paul says that the root naturally belongs to Israel, he doesn't suggest that the root IS Israel.
The Olive Tree Metaphor speaks to holiness not spirituality. The dough metaphor indicates that the entire nation of Israel is holy, even though a small remnant is spiritual. Thus Paul warns Gentile believers against bragging against the branches that were broken off.