Existential disaster: Some significant fraction, perhaps all, of the future's potential moral value is lost.

Since a whole lotta people here don't believe in morals, or at least not without so many qualifications that the average Joe wouldn't recognize what they were talking about, you need to explain this in a different way.

It all adds up to normality. The average Joe might not credit our explanations of what morality is. But such explanations are about what morality is "behind the scenes". That is, they are explanations of what stands behind our experience of morally evaluating something. But that experience itself would probably be very familiar to the average Joe.

So, when we talk about a morally value-less future, the experience that we anticipate, were we to know of this future, is just the normal one of moral repugnance that Joe would expect.