إِنَّ الْإِنسَانَ لَفِي خُسْرٍ
Indeed, humanity is surely in loss.
In 103:2, inna and the emphatic lām strengthen the assertion. Fī khusr presents loss as a condition in which humanity is situated. Al-insān has a general scope that is subsequently qualified by the exception.
Reading one particle in isolation would miss both the layered emphasis and the structural role of the following exception.