I will smite them This was not an absolute determination, but a commination, like that of Nineveh's destruction, with a condition implied, except there be speedy repentance, or powerful intercession.
Related Commentaries of Numbers 14:12
Numbers 14:12
12 I will smite them with the pestilence, and disinherit them, and will make of thee a greater nation and mightier than they.