Paul makes clear in Gal 6:16 that the fulfillment of Israel is the church. So Jeremiah's prophecies here are fulfilled in the church, not the geographical country. The geographical country will not last forever; the church will.
No, he didn't,
(Gal 6:16 KJV) And as many as walk according to this rule, peace be on them, and mercy, and upon the Israel of God.
That verse has nothing to do with Israel supplanting the Church.
(Rom 11:1 KJV) I say then, Hath God cast away his people? God forbid. For I also am an Israelite, of the seed of Abraham, of the tribe of Benjamin.
(Rom 11:2 KJV) God hath not cast away his people which he foreknew. Wot ye not what the scripture saith of Elias? how he maketh intercession to God against Israel, saying,
(Rom 11:3 KJV) Lord, they have killed thy prophets, and digged down thine altars; and I am left alone, and they seek my life.
(Rom 11:4 KJV) But what saith the answer of God unto him? I have reserved to myself seven thousand men, who have not bowed the knee to the image of Baal.
(Rom 11:5 KJV) Even so then at this present time also there is a remnant according to the election of grace.