Sorry, you cannot buy happiness with a $75,000 salary

That's a good analysis. It looks like the stress of day-to-day life is most manageable at or above $75,000, but overall quality-of-life perception continues to grow forever.

That's a reasonable conclusion. Intuitively, I would be more stressed by an unusually large gas bill, say, if I earned $30,000/year than I would be if I made $75,000/year. And at the same time, I would always be happier (feel cooler, more productive, more generous) if I had a bigger house, faster boat, or could give more to charity.

You cannot buy happiness at all. However, that isn't what they meant to say. They're trying to say that more money always means greater chance of happiness because of less stress.

As the original article stated, there's a point where more money means new stresses appear. The results of their study show that at $75k, the total of all stresses is at its least on average.

That number is wrong. These kind of studies always are due to their nature. It's a best-guess.

The point wasn't to figure out how much money people should be making, it was just a look into how money affects peoples' lives for the average person.

The point is if you think that being a billionaire will make you happy, it won't. So focus on making enough money to be comfortable and work on figuring out what actually makes you happy. Money is so rewarding a drug it drowns out everything else. This type of research helps remind people to kick the money habit.