why the ethnic nation of Israel CANNOT be abolished or replaced.

Some think that God's plan for national, ethnic Israel is abolished but this viewpoint runs contradictory to at least two reasons that Jeremiah provides in chapter 31.

1) As long as the sun, moon, and stars exist, the nation of Israel will exist as God's people.

Jeremiah 31:35-36 — Thus says the LORD, Who gives the sun for light by day And the fixed order of the moon and the stars for light by night, Who stirs up the sea so that its waves roar; The LORD of hosts is His name: "If this fixed order departs From before Me," declares the LORD, "Then the offspring of Israel also will cease From being a nation before Me forever."

2) As long as man cannot measure the immensity of the heavens, so the nation of Israel will remain as God's people.

Jeremiah 31:37 — Thus says the LORD, "If the heavens above can be measured And the foundations of the earth searched out below, Then I will also cast off all the offspring of Israel For all that they have done," declares the LORD.

It's hard to read texts like these with a plain, normal hermeneutic and conclude with any other interpretation. God states that Israel will be His people as long as the sun, and moon, and stars exist and as long as no one can reach the extremities of the galaxies and the vastness of the earth's foundations.

Paul comments on this in the book of Romans:

Romans 11:25-26 — For I do not want you, brethren, to be uninformed of this mystery-- so that you will not be wise in your own estimation-- that a partial hardening has happened to Israel until the fullness of the Gentiles has come in; and so all Israel will be saved...