Ṣāliḥ describes what is fitting or beneficial, whereas aṣlaḥ is comparative: what is more fitting or more beneficial. Calling one result best presupposes comparison with alternative possibilities.
A person's recovery from illness is a benefit. Greater comfort, a longer life, or protection from other harm might be additional benefits. Declaring every further benefit obligatory creates an ever-expanding list of purported duties. The example does not deny benefit; it tests the inference from being beneficial to being mandatory.
Human judgments about what is ultimately best are also limited by incomplete knowledge of consequences.