What is the rhetorical subtlety in Allah Almighty's saying: "Indeed, mankind is in loss," and not "Indeed, people" or "Indeed, humans"?

No specific rhetorical reason has been mentioned, but many exegetes believe that "al-insan" (man/humankind) here is a singular noun to which the definite article "al" has been affixed, indicating generality. This is supported by the exception made for "those who have believed." Al-Shanqeeti stated that "al" for comprehensiveness makes the singular a form of generality, so the meaning is "every human being." Al-Suyuti reported from Al-Qurazi that "al-insan" means "all people," with the mentioned exceptions.