Surely you aren't suggesting that the covenant of Abraham is null and void?
God made a covenant with Abraham and he has died. But many descendants of Abraham are still being born, so you could say that it's still being fulfilled.
We, however, are children of God through Jesus.
You seem to forget that although Christianity has its roots in Judaism and we share the OT, it is in fact a different religion - based on and centred round Jesus.
Christianity was seen as a Jewish sect - much as the JWs are a Christian sect today. Early Christians were persecuted; by Rome, but by the Jews as well - just as Jesus was.
Jesus was rejected by the majority of Pharisees. They did not see him as the perfect embodiment of the law; they saw him as a blasphemer. On one occasion they tried to stone him because he said "before Abraham was, I am", John 8:58.
Peter and John were thrown in prison, more than once; James was beheaded, Stephen was stoned, with Saul's approval. He himself, after meeting with Jesus, was stoned, flogged and rejected for preaching Jesus as the Messiah. Even when he spoke of his Jewish heritage and Pharisaic qualifications and momentarily silenced his critics, when he spoke of Jesus and his resurrection, they tried to kill him. He spoke out against circumcision and they persecuted him, Galatians 5:11.
The early church never said "Christianity is the fulfilment and extension of Judaism", and anyone who taught that circumcision and keeping the law were still necessary for salvation and in order to become one of God's people, was a false preacher.
I don't believe it's true to say that Christianity has replaced Judaism, as some appear to claim. Jews and Gentiles alike can now be saved through Jesus, the Messiah; who was born a Jew and fulfilled the Jewish law and prophets.
Gentiles are not required to convert to Judaism to be saved; only to convert to Jesus.