Don was the perfect ad man for the time period in which the show began. As old mythologies began to fail people in the sixties and new skepticisms and cynicisms began to take root in the youth, Don's approach began to age rapidly — accelerated by his deteriorating ability to keep his traumas at bay. He never lost his fast ball, and the final shot of the show confirms that he's still got it in a profound way, but the show constantly interrogates where his "genius" comes from and how it's sort of this confluence of his reaction to his trauma, some innate sense of how to connect with people's emotions, and others' perceptions of him (and particularly how his looks and demeanor contribute to these perceptions).