Paul is not talking about nations, he's talking to Jews and gentiles in the church in Rome. Israel as a nation wasn't cut off, but some of the branches, which means some of the people of Israel were cut out of the tree because the root was meant to inspire faith, but by rejecting faith they cut off the supply to the branches. These branches were replaced by wild branches that would receive the nourishment because they were drawing it up by faith.
So the obvious point Paul is making is that as long as the gentiles were in faith, they would receive all of the goodness promised to Abraham. But if they fell into unbelief, there was no means by which to draw on the goodness flowing from the root (promises to Abraham). As a consequence, the branches wither and become useless.
True.
This idea of a whole nation being cut off, and the reinstatement of it in the millennium is false.
Israel consists of all of Gods people who have faith in Christ, in fact God revealed His Son to all believers before and after He was born.
Joh 8:56 Your father Abraham rejoiced to see my day: and he saw it, and was glad.
Joh 8:57 Then said the Jews unto him, Thou art not yet fifty years old, and hast thou seen Abraham?
Joh 8:58 Jesus said unto them, Verily, verily, I say unto you, Before Abraham was, I am.
LA