At first glance, this paragraph seems mere restatement: in the previous chapter Monygham already determined that Nostromo was "the only possible messenger to Cayta." But something more is going on here. In this paragraph, the Devil (represented by Monygham) comes to understand that if he wants to rebuild society, he requires the participation of the People. Note that Nostromo is "really" indispensable, i.e., the People are indispendable for the practical, workaday reality of society. Note also the phrases "at last" and "the doctor surrendered," conveying Monygham's change of mind on the subject; these refer back not to the mission to Cayta, which he had already made up his mind about, but to his conversation in Chapter 3-1 with the chief engineer, in which he maintained that Nostromo was "a fool" for his fidelity to society. The Devil now realizes that he needs exactly that fidelity from the People.