I can't possibly read Gen 12:3 the way that you do. It's part of a larger statement, not an isolated sentence for one thing. He promises to make Abraham a great nation. That includes his offspring. How can God possibly make his name great and make him a blessing apart from his descendants? God also says that in him ALL FAMILIES will be blessed. Surely, God hasn't decided to exclude Israel, his own treasured people. In order for Messiah to come forth, there first had to be the people and nation of Israel who He delivered time and time again. He hasn't abandoned or replaced Israel. They are blinded for a time until the full number of the gentles has come in. Paul specifically says that he does not want us to be ignorant least we be wise in our own opinion. (Rom 11:25) In Romans 12:16, he instructs that we not be wise in it own opinion. I believe that a careful reading of Romans 11 supports this view.