[adventured]

Actualy the lower class has expanded by a few percentage points (it's not constant) due to the vast immigration of low skill workers from Latin America, while the middle class has (as you noted) been persistently moving up for decades (leading to the upper classes gaining a few percentage points).

You can see this represented over the last ~40 years based on the demographics of each bracket. All one has to do is look at the Fed's current household reports on race when it comes to wealth and income, which lays all of this out in a clear manner. White and Asian households have heavily moved up out of the middle class, while Hispanic and Black households now dominate the bottom 50% of households on income and wealth. As recently as 1980, the US was only 6% Hispanic demographically. By 2000 that had doubled to 12%, and was at 16% in the 2010 census.

Further to this point, the White demographic is now contracting. It's primarily the Hispanic demographic where population growth is occurring, and mostly via immigration.

The big middle class question for the next 20 or 30 years, is whether the huge influx of people from Latin America into the US will be able to move up over time through education and skill acquisition, and whether Black households will similarly gain.