Academic studies (as well as much of what you write here) prove that money does indeed buy happiness, except in those cases where the lack of happiness is caused by, e.g., depression, debilitating diseases, etc.

Sure, if you increase by $25k the income of two people, one making $50k and the other making $500k, the lower-paid one will see a far greater increase in happiness than the other.

However, if you double both incomes, you'll find the increase in happiness is similar.