Half a victory is not a smaller victory, it is a slow defeat that arrives inside the reward.
Nothing kept back stays the size it was when it was kept. A tolerated thing has its own aims and works on a person daily; daily friction sets the baseline state, the baseline state sets what someone is capable of feeling inside their own home, and a person can win the territory and still live badly in it.
Eyes and sides are where the damage lands: vision and forward movement, the two capacities a mission runs on. Yahweh is describing arithmetic, not punishment. A person who will not finish the clearing spends the rest of the occupancy managing what one decision would have ended.