That is not the point of the article. You're talking about quality-of-life, while the study is talking about the health of the national economy:
"The reason the decline of the middle class is important is not just about fairness. It's about the health of the economy as a whole."
So, even a hypothetical world where even the poorest person in the country has a stellar quality of life would be bad if there was no middle class, specifically because the economy would suffer. Corporate revenue would be hobbled, and so would R&D. You'd get a country of happy people that didn't do anything. A few decades of this and it would be quickly left behind by the rest of the developed world. Those few citizens who are motivated to actually do things (like found companies) will move to countries with economies that can actually support commerce.
"The reason the decline of the middle class is important is not just about fairness. It's about the health of the economy as a whole."
So, even a hypothetical world where even the poorest person in the country has a stellar quality of life would be bad if there was no middle class, specifically because the economy would suffer. Corporate revenue would be hobbled, and so would R&D. You'd get a country of happy people that didn't do anything. A few decades of this and it would be quickly left behind by the rest of the developed world. Those few citizens who are motivated to actually do things (like found companies) will move to countries with economies that can actually support commerce.