I think non-linearity was present even in earlier paper. More money adds a lot of happiness when you're poor, but marginally less as your income goes up.
The question this post is addressing is whether this effect tops out at ~75K as was found in the research from 10 years ago.
Agree but topping out is similar to saying that the slope changes. Maybe after hitting 75k, it takes another $400k to make an appreciable difference.
It may take a another $40mm to make a difference at $750k (100x for a 10x change)
The question this post is addressing is whether this effect tops out at ~75K as was found in the research from 10 years ago.