Who said he gets to remain in the spirit realm? All those who sin against God will reap what justice demands. The devil and his cronies are headed for the lake of fire....which means everlasting death.
Matt 25:31-46....
“When the Son of man comes in his glory, and all the angels with him, then he will sit down on his glorious throne. 32 All the nations will be gathered before him, and he will separate people one from another, just as a shepherd separates the sheep from the goats. 33 And he will put the sheep on his right hand, but the goats on his left.
34 “Then the King will say to those on his right: ‘Come, you who have been blessed by my Father, inherit the Kingdom prepared for you from the founding of the world. . . . .“Then he will say to those on his left: ‘Go away from me, you who have been cursed, into the everlasting fire prepared for the Devil and his angels. . . . .These will depart into everlasting cutting-off, but the righteous ones into everlasting life."
The devil was the first to defect, but he wasn't the only one. He managed to get the humans to join him by bad mouthing God and lying about him....he has been doing so ever since because his worshippers die, as God said they would, so he has to constantly recruit more.
He also managed to get a third of the angels to defect with him. What that means is that all of God's children needed to be tested as to fitness to retain the gift of life and to learn how to 'drive' free will.
Continuing life was conditional from day one. It was a simple thing to consider....obey the Creator and live....disobey him and die.
The problem was that satan invaded our domain, we did not invade his. All the issues regarding the fitness of man to serve the interests of his Creator on earth rested with their decisions as to whether they would be obedient to him or not.....that is the exercise of free will. We are not robots, and all of us have the ability to make good or bad choices. Adam made a choice in response to something his wife chose to do. Circumstance often dictate what we will do in any given situation.....some mistakes are built to last, and only God can undo them. He is in the process of doing that right now.