"All Israel" cannot mean that every genetic descendants of Abraham.
John 10:24 The Jews who were there gathered around him, saying, “How long will you keep us in suspense? If you are the Messiah, tell us plainly.” 25 Jesus answered, “I did tell you, but you do not believe. The works I do in my Father’s name testify about me, 26but you do not believe because you are not my sheep.
The important context here is time:
Roman 11:25 I do not want you to be ignorant of this mystery, brothers and sisters, so that you may not be conceited: Israel has experienced a hardening in part until the full number of the Gentiles has come in
Paul is talking about two groups of people, Jews and Gentiles, over time. In the future when the full number of Gentiles has reached, all Israel who rightfully expected the coming of the Messiah will join the number of those Gentiles who rightfully expected his 2nd coming.
Roman 11:26 and in this way all Israel will be saved. As it is written:
“The deliverer will come from Zion;
he will turn godlessness away from Jacob.
27And this is my covenant with them
when I take away their sins.”
Paul is talking about the merging of these two groups of people at a future time.