The argument in Romans 9 to 11 is not "just about the Jews with Gentiles only peripherally involved. The argument is essentially this: God has hardened Israel with the divine intent of enlarging the family of God to include Gentiles. Paul concludes by warning the Gentiles to not boast over those who have been hardened for their benefit and alludes to the clear possibility that they might fall away. Here are several texts from 9 to 11 clearly showing that this is argument not about Israel mainly, but about how Israel and the Gentiles have been brough together in the purposes of God:God will do what God will do, but He is not fatalistic. He has revealed his plan to us in His Word. In Romans 9 through 11 Paul starts with the Jews and ends with the Jews (not with the Gentiles). He explains God's plans for Israel throughout the ages. The Gentiles are only a small part of those plans, partakers of the blessings that have come to Israel. His advice to the Gentiles is basically not to be arrogant because they have been grafted into the tree.
In other words, it is not the natural children who are God's children, but it is the children of the promise who are regarded as Abraham's offspring.
Who are the "children of the promise"? Paul has already told us in Romans 4 that it is a family which includes Gentiles.
What if God, choosing to show his wrath and make his power known, bore with great patience the objects of his wrath—prepared for destruction? 23What if he did this to make the riches of his glory known to the objects of his mercy, whom he prepared in advance for glory— 24even us, whom he also called, not only from the Jews but also from the Gentiles?
What then shall we say? That the Gentiles, who did not pursue righteousness, have obtained it, a righteousness that is by faith; 31but Israel, who pursued a law of righteousness, has not attained it
"Anyone who trusts in him will never be put to shame."[e] 12For there is no difference between Jew and Gentile—the same Lord is Lord of all and richly blesses all who call on him,
I will make you envious by those who are not a nation;
I will make you angry by a nation that has no understanding."[j] 20And Isaiah boldly says,
"I was found by those who did not seek me;
I revealed myself to those who did not ask for me."[k] 21But concerning Israel he says,
Although the word "Gentile" does not appear here, "those who are not a nation" is clearly a reference to Gentiles.
Again I ask: Did they stumble so as to fall beyond recovery? Not at all! Rather, because of their transgression, salvation has come to the Gentiles
The text tell the tale - Romans 9 to 11 is not a treatment substantially focused on Israel with a concluding warning to Gentiles to not be arrogant. The matter of the Gentile is woven into the argument from beginning to end.