The idea that there is some critical level of income (once 'basic needs' have been met), beyond which income no longer improves well-being has been argued by scholars, and is intuitively appealing. However, a recent study shows that data does not support this commonly made claim. Their results suggest that subjective well-being increases with income, and that this effect does not diminish as income rises. Discrepancy with previous studies may be a consequence of employing different measures of well-being.

Note that although 100% of those with incomes >500k reported being "very happy", there were only 8 individuals in this category (out of a total of 1,014 participants).

This is honestly a very surprising result. Like you said, it is a very intuitive and appealing idea that after a certain income level, an individual's subjective happiness would level off. With that being said, though, financial difficulties often account for a large degree of stress, so I can see how the progressive elimination of such problems would only serve to increase happiness.