Why does the present planet need to be replaced after the thousand-year reign?
The present planet must be replaced because by the end of the thousand-year reign it will have used up its usefulness. The Kingdom will be interrupted and lifted off long enough for a whole new planet to appear. After the rebels are removed, a new heaven and new earth replace it.
Les teaches that after the Kingdom Age has run its course, the rejecters and rebels are taken off the scene and sent to their doom; the great white throne then sends the lost to the lake of fire, which is not hell. He explains that chapter 21's new heaven and new earth mean the Kingdom is interrupted and lifted off the planet long enough for a whole new planet to come on the scene. By the end of the thousand-year reign, this planet will have used up its usefulness, so it is replaced. Les expects the Kingdom aspects seen on this Garden of Eden earth to continue into the new heavens and new earth, which he thinks will be a carbon copy of the Kingdom described in Revelation and the Old Testament. That will be eternity, spent in a Kingdom on a new earth rather than floating on a cloud.