The seed set up after David is no ordinary wish grown to full size. The promise names a particular son, David himself, all of humanity fused into one youth and carried within a man until the day he comes forth. When a man's days are fulfilled, he sleeps with his fathers, and this sleep runs deeper than the grave. In it God lies down inside humanity and forgets, for a while, that he is God. Out of that sleep the son is raised.
The kingdom established here is fatherhood itself. The heart of this covenant is not a throne but a birth from within. The son proceeds from a man's own depths, stands before him, and by calling him Father makes him what he already was. This one son rises and shows the sleeper to be God the Father.