Isaac had faith. Thus he was a child of promise. You are mistaken to elevate ethnicity over faith.
Abel also was a child of promise before there were nations. He's in the same boat as are all people today, Jew or Gentile, who have faith.
Only being in Christ matters because He is the heir of the promise.
Well, that may be right if it weren't for those pesky promises that God made to someone that He would bring them back into a land that He gave them, that He would cleanse them, take their sins away, and there would be a Redeemer that would live among them.
Those are the mystery people that throw a death knell monkey wrench into replacement theology. And since that condition that is promised by God doesn't currently exist, then it must mean that that promise will be kept sometime in the future, right? I mean, God doesn't lie, right?
Do you have any idea who the mystery people might be?