The Almighty made an eternal, meaning forever, covenant with the people of Israel. If the covenant between the Creator with the sun and the moon can be broken, then Israel will cease being a people before Him. Mankind does not get to come along later and redefine what is meant by Israel. Jesus was speaking to a Jewish audience when he made the statement you mention. He also stated that there were other sheep not of the fold he was speaking to at that moment, which were Jews. He said about the others, that he would bring them as well - meaning Gentiles.

Even Paul stated that Gentiles were grafted into Israel as part of the commonwealth. That tree of Israel was the Jewish people of that time as they understood it - not what someone centuries later decided it meant. Christianity did not make a separate redefined tree apart from the tree that already existed. They were meant to be grafted into it. Following Jesus was never a requirement definition of who constituted Israel. That is replacement theology. Israel must be understood within its Biblical context - not what someone else thinks it means.