Viola is the novel's allegory of Liberty, which gives this sentence a larger meaning.
I.e., "in the end" the ideal of Liberty, which rests on faith in the
people as a civilizing force, is unprepared for "the out-rush of the rabble,"
i.e., the deeper instinct of destructive greed. It is this deeper instinct, as
Nostromo has it, that over the course of the book renders Viola obsolete,
replaced as a revolutionary figure by the bloodthirsty photographer in Part Three.
Note the sentence earlier in this paragraph where Viola has "disregarded
the preliminary sounds of trouble"; this too has a larger meaning relating
to the rise of labor troubles and the socialist revolt.