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.
If I knew how that sausage will be made, I'd make it myself. The point of FAI is to do a massive amount of good that we're not smart enough to figure out how to do on our own.
If humanity's extrapolated volition largely agrees that those causes are working on important problems, problems urgent enough that we're okay with giving up the chance to solve them ourselves if they can be solved faster and better by superintelligence, then it'll do so. Doctors Without Borders? We shouldn't be needing doctors (or borders) anymore. Saying how that happens is explicitly not our job — as I said, that's the whole point of making something massively smarter than we are. Don't underestimate something potentially hundreds or thousands or billions of times smarter than every human put together.
I actually think we know how to do the major 'trauma care for civilization' without FAI at this point. FAI looks much cheaper and possibly faster though, so in the process of doing the "trauma care" we should obviously fund it as a top priority. I basically see it as the largest "victory point" option in a strategy game.