Compare Monygham's later line, "You, to speak plainly, are the only man."
Both lines are allegorical, signifying that only the People can practically implement
the ruler's "purposes," i.e., dream-ideals. Thus, though on the surface
Nostromo's plaint rings false -- it is his fault that he was irreplaceable
for this mission, as in fact he just boasted to Decoud
-- on the allegorical level it holds a plaintive truth: the People did not choose
their social role.