The argument can definitely be made that the happiness difference between "comfortable and independent" and "Zuckerberg" is not all that great.
This is true. But while that is low by SV standards, the $75k threshold from the study is targeted to "median america" cost of living.
If you adjust for cost of living and for purchasing power over the 6 years since the study was conducted, you end up with quite a bit more for areas like San Francisco. In fact, you'd probably be looking at more like 175-185k. [1]
1. http://www.advisorperspectives.com/dshort/commentaries/Happi...
Not that I don't want to work anymore, but being able to leave my programming job and becoming a dog breeder would make a lot of difference. And take month longs vacations to travel whenever I wanted.