"Do you think they believed that humanity was tracking onward and upward? That the future was promising?:

I don't know Whitman well, but I'd guess he did, and probably Stevenson did.

"That's not my proposition, however. It's

'...sustained by a belief in a better future--material and/or spiritual.'"

I didn't focus on that enough. I still don't think a yea-sayer would have to expect a "better future" even if just materially. But probably yea-sayers usually would expect a "better future" of some sort.

I think I agree with what you say about one's children.