Much of this Abraham and Sarah narrative is not about the delivery of the promised child. It is about the struggles, foibles, and setbacks that come along the way. Life is not a series of yay God days, one after the other for most people. We can easily identify with Abraham and Sarah in their times of testing, waiting, struggling, questioning, and nearly giving up hope. The Bible is clear about that.
The biblical record is also clear about God's good will and trustworthy nature in those times of waiting. God will make good on the good things he has promised.
