It appears that "so that God may be all in all" is saying "so that God alone will rule and God will rule alone":
1Co 15:28 ISV But when everything has been put under him, then the Son himself will also become subject to the one who put everything under him, so that God may be all in all.
But then we see the inhabitants of New Jerusalem being said to "rule and reign forever":
Rev_22:5 DRB Rev 22:5 And night shall be no more. And they shall not need the light of the lamp, nor the light of the sun, because the Lord God shall enlighten then. And they shall reign for ever and ever.
So why does Jesus need to abdicate in order for God to "be all in all" but the populace continues to reign alongside God?