Another reason I think there has always been one plan for one people comes from Romans 5.13-14: “For sin indeed was in the world before the law was given, but sin is not counted where there is no law. Yet death reigned from Adam to Moses, even over those whose sinning was not like the transgression of Adam, who was a type of the one who was to come.”
Paul’s point here is that even before the Law was given, there was a Law, or else men would not have died. So the Law given to Israel was not something completely new, just revealed in a clearer way. In other words there is much more continuity than first meets the eye throughout the Bible. And Paul says you were supposed to know that Adam was pointing to someone future. The OT was not (mainly) about Israel, but mainly (only?) about Jesus.