Macbeth should have realised that the line of kings from Banquo's son onwards would seem perpetual. Those members of the audience with a knowledge of history would be aware that the monarch at the time the play was performed, James I, was directly descended from Banquo's line. In this way Shakespeare is utilising some (but certainly not all) of the historical events upon which the play is loosely based. Also, as Macbeth had disrupted the Natural Order and the Divine Right of Kings which were strong beliefs of the time, it was to be expected that his hold on the throne would be brief, and his audacious plan to subvert God's Will would be punished.
In the light of this vision, the prophesies as given to Banquo in Act I scene iii become clearer in their meaning-
Lesser than Macbeth, and greater.
Thou shalt get kings, though thou be none.
Banquo himself does not live to be king, but his bloodline does.