It's just a combination of a) I don't see anything that could be called as a definite source of inspiration predating D&D in a way that, say, Conan is an inspiration for Barbarian Hero archetype and without it I cannot really eliminate a possibility d&d may have codified the modern image of "sword & board fighter" from various preexisting sources and b) it kinda feels to me like this thread is turning into "if we can name a single example older than D&D, D&D gets no credit for influencing popculture here" with no nuance or acknowledgment of the idea of, to use tvtropes terms, trope codifiers. See also discussion about the Lich.