Did Aaron Copland steal half his career from a few measures by Debussy?

Wow! I don’t know if he thought specifically about this passage, but, now that you pointed out, I have to agree that most of the pieces I heard from him do seem to be mostly dependent on characteristics contained by it. Debussy was certainly a key influence in Copland’s style. Taking into account the fact that these few measures seem to encapsulate so much of his music, I think it would be fair for us to assume that, when “charting his territory”, to borrow a phrase now from Harold Bloom, he probably chose a more contained and limited expressive breadth than that chosen by Debussy.

Of course, Debussy himself, when struggling with Wagner’s influence, did choose as well a more restrained scope of harmonic development, although he largely compensated for it through interesting harmonic innovations and through an extensive effort in exploring the possibilities of tone color. The two instruments I play are the piano and the flute, and it’s no coincidence that Debussy is usually considered the major reference in modern music for both of them, and it’s probably no coincidence either that he is my favorite modern composer. Debussy was probably a lesser composer than Wagner, but, nonetheless, he sometimes feels more expressive to me in some respects (although only in some respects). I’m not sure to which extent Copland was able to compensate for his decision, though.

Ultimately, I don’t think it’s necessarily negative for artists to chart for themselves narrower "territories" than those chosen by their antecessors. However, if every new generation were to keep doing that, as I feel is most often the case in the twentieth century, it is sure to reflect negatively in the large scale development of the art in question.