I'm not certain what you base your confidence on given trackable economic and social mobility markers don't support it. Its fascinating that apologists default to some combination of postmodern argle-bargle over defining the term "middle class" and pointing to modest economic successes in what until recently were unambiguously 3rd world countries when a discussion of the middle class in America comes up. I honestly do not understand the perceived relevance of economic outcomes in ex-soviet countries in this context, unless the goal is to provide some kind of cover for neoliberal economic theory, which also seems nonsensical unless one is some form of pundit or politician.
50 years ago a single income family being able to afford to own their home, at least one car in the driveway, school their children, and comfortably save for retirement was the generally accepted definition of "middle class". I honestly can't be bothered to even look up what passes for a more modern definition given regardless of where those goalposts get planted someone's going to argue anything north of abject poverty is "middle class". After having sat a few hundred iterations of that debate I think I can feel my soul trying to leave my body at the mere thought of doing another lap.
One of the things that freaks me out the most about this kind of cognitive-dissonance-fueled shit flinging contest is I am deeply mystified by the notion that there's even anything controversial here. Rural America is not ok. The average cost of a house and a medical degree in the US are approaching parity. The current rise in populism also didn't spontaneously arise, it's a reaction to economic pressure (among other things). How much worse does it have to get before the conversation pivots from "is there a problem" to "k, maybe we should work on some of this"?