For instance, TDT implies that one should one-box even in a version of Newcomb's in which one can SEE the content of the boxes. But would a human being really leave the other box behind, if the contents of the boxes were things they REALLY valued (like the lives of close friends), and they could actually see their contents?

Probably not, and thus s/he would probably never see the second box as anything but empty. His/her loss.