Not so. The "his people" are the "Israelites" of which Paul is one. Read it:
"Hath God cast away his people? God forbid. For I [Paul] also am an Israelite, of the seed of Abraham, the tribe of Benjamin" [Romans 11:1]
God's people are not the same as Paul's people. Some are; but not all. Paul's people, his lineage, as he says, are those of the physical circumcision, the Jews.
God's people, all believing Jews and Gentiles, are those whose hearts have been circumcised.
"For circumcision verily profiteth, if thou keep the law: but if thou be a breaker of the law, thy circumcision is made uncircumcision." -- Romans 2:25
And which Jew or which Gentile can keep the law perfectly?
NONE.
So the physical circumcision means nothing and profits nothing.
But he is a Jew, which is one inwardly; and circumcision is that of the heart, in the spirit, and not in the letter; whose praise is not of men, but of God." -- Romans 2:28-29 "For he is not a Jew, which is one outwardly; neither is that circumcision, which is outward in the flesh:
“Hath God cast away his people? God forbid. For I [Paul] also am an Israelite, of the seed of Abraham, the tribe of Benjamin” [Romans 11:1]”
And continuing:
Romans 11:2-5 God did not reject his people, whom he foreknew. Don’t you know what the Scripture says in the passage about Elijah—how he appealed to God against Israel: “Lord, they have killed your prophets and torn down your altars; I am the only one left, and they are trying to kill me”? And what was God’s answer to him? “I have reserved for myself seven thousand who have not bowed the knee to Baal.” so too, at the present time there is a REMNANT chosen by grace.
Whom do you think Elijah was speaking about when he said “they have killed your prophets and torn down your altars”?
UNBELIEVING Jews. They perished. Their bloodline didn’t save them. Only the believers were saved.
So, of course, God did not cast away His people, the remnant of believing Jews, which later included a remnant of believers of all nations, in Christ.