I feel that a good FAI designer shouldn't dismiss these objections so easily. The toe-stubbing problem is much simpler than preserving human existence. To solve the toe-stubbing problem, you just need to correctly order two outcomes A and B. To preserve human existence, you need to order outcome A against all possible outcomes B that a powerful creative intelligence might come up with. Any solution to the bigger problem that doesn't allow you to solve smaller problems is very suspicious to me.