No one has suggested that Jews, or anyone for that matter, who do not have faith in Jesus Christ are grafted into anything. It seems that those who believe in supersessionism have a blind spot. Just because in this current dispensation, or era, Israel does not see who Jesus Christ is doesn't mean that in a future dispensation, or era, that God won't open their eyes.
What this series of threads is pointing out is not just the error of supersessionism, but also how this erroneous belief became popular. As we see it did not originate during the Apostolic Era. It started to emerge after interpretation of Scripture shifted from literal to allegorical. Justin Martyr got the ball rolling by promoting the idea that all promises God made in the past were now meant for the church.
It may seem off the mark, but if supersessionism is true why are the Jews occupying their God given land and not Christians? I understand why the RCC is always trying to get control of Jerusalem (in the name of peace of course). They are supersessionist and amillennial. I'm sure it's part of the reason they profess such a close tie to the muslims. What is surprising is to see the Reformed, who separated from the RCC with the battle cry "Sola Scriptura" ignore the literal words of Scripture that indicate God will return the Jews to the land He gave them and will open their eyes.
Rom. 11:25 ...that blindness in part has happened to Israel until the fullness of the Gentiles has come in.
That being said, I don't believe you can make the statement that "it did not originate during the Apostolic Era". There are a number of not only onesy verses, but three chapters of Romans is devoted to it.
I take NO position of eschatological understanding except to say a person is either in Christ or not in Christ. A person is a believer or unbeliever. A child of the light or a child of darkness. Righteous or unrighteous. This is what the Kingdom of God is all about. We are sojourners. And those who believe in God are part of this family with all the honor accorded by God the Father and His Son.
Those who do not believe in the Son are not part of this family. And, as you've stated, it is God that opens our eyes and ears to see and hear the truth. It is possible that someday the physical Israel will have their eyes opened to the truth and come to be saved. But when, and if, that day happens God will save them just like He did with Abraham, Moses, Noah and all the rest-to His great glory and honor. But until that occurs, as the scriptures teaches the wrath of God is upon them, for they have rejected the Son.