Mrs Gould suddenly becomes a commanding, intervening matriarch here. It is not entirely out of character; she was "fastidious" in her own engagement , and had subtly expressed doubt in the propriety of Decoud's, when told he had become a journalist. Allegorically, altruism requires above all an upper-class position, as the novel has suggested several times with relation to Mrs Gould, not least here, where her upper-class title is deliberately invoked.
On the allegorical level, Nostromo's love for Giselle is the People's quest
for a society no longer based on conquest and oppression. In altruism's eyes
it is a "situation" that "lends itself to --" several unnamed
bad things. For one, failure and the complete breakdown of society, since the
quest is futile. But even if successful, the resulting classless society
would leave no room for altruism. Therefore altruism, which began with the ideal
of uplifting the People from oppression, now makes the "immense decision"
to force the People into allegiance with the oppressive system (represented
by Linda), for the sake of preserving the material society it doesn't like and
thus its own continued role.