You are aware that The Culture is explicitly not a descendant of humanity. Culture agents visit Earth in a short story set after Consider Phlebas and it's during the Middle Ages.
As for your alleged empirical fact -- the implication is that almost no-one is wealthy, which is either manifestly untrue or your definition of happiness isn't useful.
Culture agents visit Earth in a short story set after Consider Phlebas and it's during the Middle Ages.
I'm not familiar with any medieval Culture encounters (and I don't see any in Wikipedia's description of the Culture's fictional history[0]), but there is a story, "The State of the Art", where Culture agents show up in the 1970's, which supports your point about the Culture explicitly not descending from humanity.
As for your alleged empirical fact -- the implication is that almost no-one is wealthy, which is either manifestly untrue or your definition of happiness isn't useful.