Denying that the optimal benefit is obligatory does not negate Allah's justice or wisdom. Justice concerns right judgment and the absence of wrongdoing, whereas grace concerns gifts and beneficence beyond entitlement. Keeping the two separate prevents every ungranted benefit from being labeled injustice.
If an additional advantage was possible but not granted, its mere possibility does not show that the servant had an enforceable entitlement whose nonfulfillment would be wrong.
Hope, prayer, and responsible effort nevertheless remain appropriate; divine decree is not offered as an excuse to abandon legitimate means.