Why does the royal line from Abraham and David continue into eternity?

Les teaches that David becomes the beginning of a royal family that will not merely lead through generations of this present existence, but continues all the way into eternity. The reason is that out of the line of Abraham and David would come this government, and it will not be a democracy but a monarchy ruled by the King of Kings and Lord of Lords. This King will not be a despot or totalitarianist, but benevolent; His kingdom will be righteous, fair, and glorious. Les explains that this prospect was laid out in the Abrahamic covenant, and the rest of the Old Testament from Genesis 12 onward rests on it. God also promised David a royal family leading up to the King, and that is the government.