Believers who survive the Great Tribulation and enter the Millennium will help repopulate the earth. What happens to these believers when they die in the Millennium? They obviously can’t/won’t go to the New Jerusalem, where else could/would they go?
The disposition of millennial believers is not specifically addressed in the New Testament just like the disposition of New Testament believers is not specifically addressed in the Old Testament. Recently I’ve been speculating on the possibility that at the outset of the Millennium a third testament will emerge to deal with the spiritual aspects of life on Earth in the Millennium, just like what we call the New Testament emerged after the Church was born to deal with the spiritual aspects of our life.
From what little we can see, it appears that spiritual life for believers in the Millennium will be much different than it has been for us during the Church Age. Therefore it makes sense that some kind of instruction in this regard will be provided when the time comes.