I'm saying that she has an obligation, whatever she does, not to think about her society as a pit of crocodiles (except hypothetically, for abstract arguments, etc - never in semi-conscious daily thinking, as a matter of "attitude"), because that'll only increase the class divide and its problems. Society is affected by its members' perception of it, and if everyone just wanted to maximize safety for themselves and their families... why, that society would be utterly helpless!

This sounds much like a tragedy of the commons. Remind me again what the rational response is if one has little hope of organizing measures to overcome it.

If the middle class just evacuates from everywhere where they have any contact with the "underclasses", so that the latter are left in utter and visible isolation, like the "Untouchable" castes in India...

Where is the evidence that living with the underclass benefits the underclass more than it hurts the middle class? In case you haven't notice the underclass has due to social changes nearly fully assimilated the wrecked working class that existed in the United States at one point and its cultural norms and dysfunction are spreading and becoming worse. I believe the non-blue collar middle class is next and nearly all social indicators seem to be moving in that direction.

For a brief overview of just how far the cultural class divides have grown I suggest reading Charles Murray's "Coming Apart". Why group such different cultures under the marker "American culture" in a sense beyond geographical designation?