In context, the metaphor in the third sentence ('it was only a subplot in a historical drama of time, place, and people that had been playing for thousands of years') chiefly serves to:

While the metaphor does hint at the historical backdrop of the United States, its primary purpose is not to highlight the nation's age but to illustrate the expedition's role within a much broader historical framework. Therefore, this choice overlooks the metaphor's intention to situate the expedition within a larger context rather than merely emphasizing the country's timeline.

The metaphor does not critique the literary merit of Lewis and Clark's writings. Instead, it serves to place their expedition within a grander historical narrative, suggesting that their contributions are part of a continuum rather than diminishing their literary significance. This interpretation misreads the metaphor’s intent.

Though the metaphor suggests that the expedition is part of a larger story, it does not intend to trivialize its accomplishments. Rather, it aims to provide a richer context for understanding its importance in relation to the broader historical developments of time, place, and people, thus respecting its contributions.