One factor you didn't mention is that significantly harder working people might need to spend more income to maintain their productivity. E.g. traders working 80-hour weeks might spend more on food because they don't have time to cook, more on clothes because their job requires signaling, more on airplane tickets because they have short-notice time-sensitive tasks requiring travel, etc. But there's still no reason this should work out to anything like proportional giving, so your point stands.