Does estimating the worst-case scenarios when calamities strike, as mentioned in Aid al-Qarni's book "Don't Be Sad," contradict the Prophet's (peace be upon him) love for optimism and his dislike for pessimism?

The pessimism mentioned in the hadith (definition) is a form of ill-conjecture about Allah, weakness in reliance upon Him, and attachment to delusions. As for the good omen (fa'l), it is the righteous word and a form of good conjecture about Allah. However, expecting the worst in order to take precautions and prepare for difficulties is not blameworthy; rather, it may be religiously commanded and differs from the prohibited pessimism.