The Millennial reign is the last "Day" of this present creation, and
after The Sabbath Millennium is the 8th Day, "that Great Day of New Beginnings", which is oracled in the Law. The sign of it is circumcision on the 8th day, for male Jews, for it is when the Adam nature -multiplied by the Adam seed- is cut off from multiplying forever: but then, for a million years -forever and forever- Israel exists as the nation of God on earth, where forever and forever the kingdom is restored to what Adam was made for, which was to bring forth sons of God by multiplying the seed within, so as to "Plant the heavens" and be that temple not made with hands for the Glory to indwell.
So God's plan is for sons of God to be born of human beings to plant the heavens forever. If Adam had not fallen he would have brought forth the ordained seed from his loins and been changed after he lived his thousand year day,to the heavenly body that does not marry nor give in marriage but is like and one with the angels in heaven.
God is still going to do what He made this earth for, so why would He abandon His one plan? He always intended to have a temple not made with hands of human being sons, but He made His heavenly created temple before He created Adam male and female to be the temple not made with hands: because His created heavenly temple is setin the sun, and the sun will endure forever, then His heavenly temple will endure forever, and the temple not made with hands will endure forever -and multiply and plant the heavens- and so why would His temple in Jerusalem not endure forever? Even if it is destroyed and built again, God still calls it the temple, and Israel itself is to always have a temple below, to prepare sons to build the City of God always, above.
Adam was created to live a thousand year day and be transformed, and all his seed were to have also been transformed after their day was ended, and in the millennial reign, there will be no death but for sinners who die accursed at age 100, the rest will live their day, and be transformed, but their children will also live their day and be transformed, and so on.
Since a "day" is the thousand year span for a human to live and multiply and then be changed, the thousand generations God has promised is a million years....and it will be perpetual.
Deu 7:9 Know therefore that the LORD thy God, he
is God, the faithful God, which keepeth covenant and mercy with them that love him and keep his commandments to a thousand generations;
1Ch 16:15 Be ye mindful always of his covenant; the word
which he commanded to a thousand generations;
Psa 105:8 He hath remembered his covenant for ever, the word
which he commanded to a thousand generations.
Isa 51:16 And I have put my words in thy mouth, and I have covered thee in the shadow of mine hand,
that I may plant the heavens, and lay the foundations of the earth, and say unto Zion, Thou
art my people.