Psalm 132:11. "The Lord swore to David a sure oath from which he will not turn back: 'One of the sons of your body I will set on your throne.'"

God's promises are often shown to be true in the shorter run and then, more fully, in the much longer run. This is because everything is cyclical, so prophecies and promises can seem to have been fulfilled in one cycle, but then in a later one they are fulfilled much more completely. David's son Solomon did take the throne after David's death, so this oath of God's was fulfilled in its literal sense almost immediately, but the real fulfillment of this oath came when Jesus died on the Cross, His earthly throne; and rose again, having defeated death. That was a fulfillment that no one in David's time could have foreseen; one that is so radically different from what we would expect that even now many fail to see Jesus as the Son of David who rules forever. Jesus truly is King of the Universe, but a lot of His sentient beings (that is, us) don't recognize that yet, which only means that we are in another cycle. Jesus' kingship will eventually be revealed very clearly to all, in a way that will track with Revelation and other prophecies, but will also be very different than we expect, just as His first coming was.

I think God lets us see the more immediate and more literal fulfillments of His promises so that we both come to trust Him and come to see that only He can satisfy us completely. Using Solomon as an example of this: Solomon took the throne of his father David in fulfillment of God's oath, and Israel became powerful beyond imagining under his rule. Solomon was uniquely wise, because God gave him wisdom, and was revered and famed throughout the world. So in the short-term, it very much seemed that God's oath to David had been fulfilled, and His blessings were being poured down upon Israel. But then Solomon let his pride and human weaknesses take over, and Israel eventually suffered greatly.

This is the cycle of human history, it always has been and always will be this way with human leaders and earthly nations. But because they knew God and had seen how His blessings are bestowed, the Jewish people retained hope in His promises. They learned that only God can be their "forever" rightful king, and held on to the promise of a Messiah, a son of David who would rule from his throne for all of time.

This is how we all can come to believe and hope, even when our human leaders disappoint us. They always will, but when we have tasted the goodness of having a good leader even for a short time, we can hope for the goodness of having a good leader for eternity. The only one who fits that bill is God himself, so we turn to Him and His promises, which will eventually be fulfilled at exactly the right time, in the most perfect way!