I doubt human value is particularly fragile. Human value has evolved and morphed over time and will continue to do so. It already takes multiple different forms. It will likely evolve in future in coordination with AGI and other technology. I think it's fairly robust.
Like Ben, I think it is ok (if not ideal) if our descendants' values deviate from ours, as ours have from our ancestors. The risks of attempting a world government anytime soon to prevent this outcome seem worse overall.
We all know the problem with deathism: a strong belief that death is almost impossible to avoid, clashing with undesirability of the outcome, leads people to rationalize either the illusory nature of death (afterlife memes), or desirability of death (deathism proper). But of course the claims are separate, and shouldn't influence each other.
Change in values of the future agents, however sudden of gradual, means that the Future (the whole freackin' Future!) won't be optimized according to our values, won't be anywhere as good as it could've been otherwise. It's easier to see a sudden change as morally relevant, and easier to rationalize gradual development as morally "business as usual", but if we look at the end result, the risks of value drift are the same. And it is difficult to make it so that the future is optimized: to stop uncontrolled "evolution" of value (value drift) or recover more of astronomical waste.
Regardless of difficulty of the challenge, it's NOT OK to lose the Future. The loss might prove impossible to avert, but still it's not OK, the value judgment cares not for feasibility of its desire. Let's not succumb to the deathist pattern and lose the battle before it's done. Have the courage and rationality to admit that the loss is real, even if it's too great for mere human emotions to express.
The problem with this logic is that my values are better than those of my ancestors. Of course I would say that, but it's not just a matter of subjective judgment; I have better information on which to base my values. For example, my ancestors disapproved of lending money at interest, but if they could see how well loans work in the modern economy, I believe they'd change their minds.
It's easy to see how concepts like MWI or cognitive computationalism affect one's values when accepted. It's likely bordering on certain that transhumans will have more insights of similar significance, so I hope that human values continue to change.
I suspect that both quoted authors are closer to that position than to endorsing or accepting random value drift.
I haven't yet been convinced that my values are any better than the values of my ancestors by this argument.
Yes if I look at history people generally tend to move towards my own current values (with periods of detours). But this would be true if I looked at my travelled path after doing a random walk.
Sure there are things like knowledge changing proxy values due to knowledge (I would like my ancestors favour punishing witches if it turned out that they factually do use demonically gifted powers to hurt others), but there has also been just plain old value drift. There are plenty of things our ancestors would never approve of even if they had all the knowledge we had.