Considering that there are probably a great many discernible levels of intelligence above that of our own species, and that we were not especially designed to build FAI, do you have reasons to think that the problem difficulty and human intelligence are within what seems to me to be a narrow range necessary for success?

Humans have already invented so many astounding things that it seems likely that for most things that are possible to build in theory, the only thing preventing humans from building them is insufficient time and attention.