I like to look at it as the power of money to most people isn't what it adds to your life, it's what it takes away. No more fears about bills, no more worrying about being broke or stressing over the monetary concerns. That/those are the first plateau. Once those things are gone some people are going to be a lot happier, but sometimes money wasn't the actual cause of your unhappiness; just one of many causes amidst the pile. You might be more happy, or less stressed, neither, or both.

These concepts are nebulous at best, but I'm curious how one measures an increase in happiness versus a decrease of stress. Likely a fair amount of comorbidity at play.