Antonia's line is beautifully written, its rhythm capturing her breathless passion.
"Aspirations" also means "breathing," creating a sort of grim pun on Decoud's impending death. But in what follows, we must remember that Decoud's "aspiration" is not just to love her, but to carry her out of Costaguana. That is why his teeth are clenched and his bow is distant. He has not yet won this point.
On a deeper level Antonia's line also conveys the essential defense of idealism:
i.e., "why should anyone consider ideals futile?" Note that the defense
is phrased as a (supposedly rhetorical) question. It is not a positive statement
that ideals are realizable, merely a negative warning against considering
them unrealizable, i.e., indulging in skepticism. Thus again we see Antonia
as the voice of pure idealism, arguing for the good fight regardless
of the possibility of winning, as above with her remark, "We are labouring
to change all that."