This is just factually incorrect. An idea never posited until the modern era. Here you have Ignatius of Antioch (who knew and learned from St John) writing around the year 110:
“See that ye all follow the bishop, even as Jesus Christ does the Father, and the presbytery as ye would the apostles; and reverence the deacons, as being the institution of God. Let no man do anything connected with the Church without the bishop. […] Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. […] Whatsoever [the bishop] shall approve of, that is also pleasing to God, so that everything that is done may be secure and valid.”
There are an almost limitless supply of other pre-Constantine references to "the Church" and meaning it explicitly in the way in which I am referring to the Church. I mean, Our Lord himself creates the Church by ordaining the Apostles and placing Peter over them. "Feed my sheep." "What you bind on Earth is bound in Heaven...". etc.
Perhaps we're talking past each other to a certain extent. I am not saying Jews can never be accepted into the Church or that they lost salvation simply by once being Jews. You are correct in that we are all grafted into the vine and should claim no special privilege for having been grated on. And I definitely do not suppose that anyone being grafted in "takes the spot" of someone else.
What I am saying is that rejecting Christ, which Jews must do or else they are no longer Jews, means that you cannot be grafted on. That is the absolute bare minimum for being seen as a good servant of Our Lord. Again, they can be grafted on at any moment by abandoning Talmudic Judaism and accepting authentic Christian teaching. I am not claiming they are cast out and can never return. But they actually have to accept Our Lord for that return to happen. As of now, that is not the case and probably won't be until the end, if you accept that interpretation of Revelation.
To head off the old covenant still being in place argument, I must ask how a Jew today can ever faithfully uphold the covenant without a Temple? Why would God allow the Temple to be destroyed if he expected the Jews to still faithfully follow the Mosaic Law? Isn't it much more reasonable to see the destruction of the Temple (and it's never having been rebuilt for 2000 years) as a sign from God that the old covenant can no longer be fulfilled by humans. The only way is through Our Lord Jesus Christ