The Abrahamic Covenant was instututed by God as the means by which he would bring the blessing of salvation unto all who would qualify as Abraham did, by receiving the promised Messiah yeshua, so that all who place faith in Him will be saved!Paul does teach that the Abrahamic covenant was set up in the past. But he does not teach that this was a covenant of nationalities (of Gentiles on one side and Jews on another). The Object of this covenant is the Promise, the Seed (singular) who is indeed a Jew (to the Jew's first, and then the Gentiles). But to Abraham there was no Israel (Jacob/Israel was one of Abraham's grandchildren). Paul goes back to that covenant and declares (although you disagree) that the Seed is Christ and the children are those who believe.
I also believe that God has a purpose for Israel (as a nation) that will come to pass in the future. A belief that God has a future for Israel (as a nation) is not dependent on restricting the Abrahamic Covenant to Jacob and his descendants. Abraham was not a Jew. He was not an Israelite. Abraham was not even the "father" of the Jews (in terms of sole lineage). That would be Jacob (whom God chose and changed his name to Israel, and whose descendants are the twelve tribes of Israel).
You can't go back to Abraham and restrict that covenant to a future (to that time) people because it would exclude Abraham himself. Abraham had Isaac. God established his covenant (within the Abrahamic Covenant) with Isaac (for it is through Israel the Seed will be delivered). And God established his covenant with Jacob (Israel). Again, within the Abrahamic Covenant. But the Object of the Abrahamic Covenant is not provision for national Israel, but salvation to the world. And if you cannot understand that simple point, then you are wrong even in your dispensationalism.