According to 2 Samuel 7:16, David will always have a descendant on the throne of Israel. This promise appears to be unconditional. Indeed, many interpreters look at passages like the one made directly to David as being fulfilled in Jesus. Well, yes and no.
One big problem with the view that God’s promise to David is unbreakable is the fact that, between Zerubbabel (not really a king) and Jesus, there was a really long dry spell of no Davidic kings. Half a millennium!