If we can't stop entropy, then we can't stop entropy, but I still don't see why our descendants should be less able to deal with this fact than we are. We appreciate living regardless, and so may they.

Surely posthuman entities living at the 10^20 year mark can figure out much more accurately than us whether it's ethical to continue to grow and/or have children at that point.

As far as I can tell, the single real doomsday scenario here is, what if posthumans are no longer free to commit suicide, but they nevertheless continue to breed; heat death is inevitable, and life in a world with ever-decreasing resources is a fate worse than death. That would be pretty bad, but the first and last seem to me unlikely enough, and all four conditions are inscrutable enough from our limited perspective that I don't see a present concern.