The servant's grief would lead to a greater good: the forgiveness and reconciliation of sinful humanity. Note also God’s good plan to prosper and exalt the servant.
when His soul is made a guilt offering: The servant's suffering served as a substitute for others, similar to animal sacrifices in the Temple.
Having many descendants and enjoying a long life are rewards for living wisely and righteously (see Proverbs 3:2; 17:6; 20:7). Because the servant trusted his fate to God, he would receive eternal rewards from God, who vindicates the righteous (see Philippians 2:9–11).