It's worth noting that while the curve flattens above a threshold, it doesn't level off completely at that threshold, there is still a positive correlation, just a smaller one.
And when is that exactly? It definitely isn't making (unadjusted for inflation) the $70k that study suggests.
People are happy when they are secure and unhappy when they are insecure. Who can you name is secure in all of their physical, social, mental, spiritual, etc needs right now?