I think you’re mixing metaphors. If you earn a six figure income in most of the US, you’re decidedly not in the lower class. But that lifestyle is decidedly not the same as people in the upper class. There are a lot of shades to being “rich”, and much fewer to being “poor”, but you know when you’re rich and you know when you’re poor. If you know you’re not either, you’re in the middle class. This describes a great portion of the US population.

Dividing the world into two groups, the upper and under, seems to be more about politics and power, and through that lens, many “rich” people are still in the “underclass” as well.