Sea Breeze, I believe in "supplemental" theology in the sense that the Israel concept was enlarged to include the nations, i.e., Christians. There is a debate as to what the trunk symbolizes. Jesus is the root, as many scriptures indicate (cf. Is. 11:10; 53:2; Rom. 15:12; Rev. 5:5; 22:6; cf. Rom. 11:17, 18). The wild olive branches would be the people of the nations, i.e., Christians. Natural branches, to be grafted in again, would be the Jewish remnant.
Originally, Israel was in a privileged position: "who, as such, are Israelites, to whom belong the adoption as sons and the glory and the covenants and the giving of the Law and the sacred service and the promises; to whom the forefathers belong and from whom the Christ [sprang] according to the flesh" (Rom. 9:4, 5 NWT). Now they must share these privileges with the people of the nations: "For there is no distinction between Jew and Greek, for there is the same Lord over all, who is rich to all those calling upon him." (Rom. 10:12 NWT)
Israel would have been the premier nation if they stayed the course. Now only a remnant will be saved (Rom. 10:22, 23; cf. Is. 10:22, 23). While Jesus was on earth, they claimed their superiority because of being offspring of Abraham (Luke 8:39, 44). But Paul explained Abraham was righteous, not because of circumcision, but because of his faith (Gal. 3:6, 7). On this basis Christians also become “offspring” of Abraham, because of their faith (3:8, 9, 29).