Measuring "class" by income distribution measures like this kind of misses the point; more accurately this is "Most Americans no longer fit a particular definition of 'middle income'". Class distinctions are about significant qualitative distinctions, not points on a simple quantitative distribution. A reasonable upper/middle/lower class distinction could be made where the upper class has capital assets that they can…
If you have to work for a living, your life is similar to everyone else who has to work for a living. Sure you may have more toys than others, but the things you think/worry about from day to day are similar.
If you are in the class that relies on their assets for a living, you think about entirely different things, and your world view and lifestyle are entirely different. If you ever get a chance to hang around people like this or talk with them, do it--it's eye opening. It's like having an alien encounter.