It seems very difficult to argue for a "flat" discount function, even if (as I can do only with some difficulty) one sees things from a utilitarian standpoint: I am not indifferent between gaining 1 utilon right away, versus gaining 1 utilon in one hundred years.

Probing to see where this intuition comes from, the first answer seems to be "because I'm not at all sure I'll still be around in one hundred years". The farther in the future the consequences of a present decision, the more uncertain they are.