"Fire and Ice" is set up as a choice between fire and ice. Which force will bring about the end of the world? Based on the wisdom gained from his experience, the speaker decides that desire and the other forces of "fire" would probably bring about the destruction of the world first. "Fire," after all, is the realm of the passions, which are spontaneous and impulsive. But the cool deliberation of "ice" would be no less effective at bringing about destruction. The speaker makes a choice but avoids choosing one over the other. How'd he do that?!
The end of the poem reveals that the question of whether fire or ice will destroy the world is a flawed and ineffective way of thinking about these two elemental forces.
Frost reinforces a false and overly simplified dichotomy by refusing to consider that fire and ice can mix.