As mentioned, it’s a mistake to tie social class to income.
Back in the middle ages, you were either a commoner or a nobleman. Nobles were rich, commoners were poor. But wealth didn’t mean what it means now, because the only method of generating wealth was to skim the commoners. You had lands, the commoners worked the lands, and gave you a cut in return for you not lopping off their heads with a sword. Being a nobleman wasn’t about wealth, it was about military status. A nobleman was a person who was a full time warrior. And his job wasn’t primarily about lopping the heads off commoners, because it was generally futile for a commoner who had to work for a living to challenge a nobleman, but rather fighting other nobles over who had the right to steal from which commoners.
Now, the feudal system lasted a while, but an interesting thing started to happen. Certain commoners started to amass wealth, except NOT at the point of a sword. They traded, or made stuff, or somehow mysteriously cheated their way into wealth. It wasn’t honest, like how noblemen attained wealth (by stealing), or how peasants attained wealth (by backbreaking agricultural labor), it was low, it was petty, it was grasping, it was cheating. These people were the new “middle class”. They weren’t peasants, but they weren’t nobles, they were something else.
Eventually, more and more peasants left the agricultural work force, and became proletarians, or working class. That is, people who owned no land or property, and lived hand to mouth as industrial workers. So a shopkeeper would be middle class, a teacher would be middle class, a professional like a doctor or lawyer would be middle class, but a guy who worked in a factory is working class.
But the distinction between the merely wealth bourgeois and the upper class has pretty much disappeared. Back before the civil war, there could still be a feeling that a man whose wealth came from owning a plantation was somehow superior to an equally wealthy man who owned a factory or engaged in trade. But that’s pretty much gone, even in Europe. The upper tier of the middle class merged with and/or absorbed the old upper class. The old upper class values and virtues were totally replaced by bourgeois values and virtues. Bill Gates, Donald Trump, Sam Walton, and Warren Buffet aren’t aristocrats despite being among the richest people in the world.
The biggest distinction between upper middle class and “rich” is that upper middle class people work for a living, while the rich own and manage property. But this is kind of an arbitrary distinction. Bill Gates certainly worked at Microsoft, he didn’t just sit back and collect dividends. There are plenty of “old money” people who work. And calling an executive who manages a company but doesn’t own it “middle class”, while the owner is “rich” is arbitrary, especially since so many businesses aren’t owned by one guy, but are publicly traded. So that hired manager doesn’t work for a rich guy, more typically he works for thousands of stockholders who are almost all less wealthy than he is.
And of course, the old distinction between the peasantry and the middle class is meaningless as well. Even migrant farm workers aren’t peasants, although they might have been peasants in their home country. So the only meaningful distinction would be between working class/proletarians and middle class. But that’s kind of slippery too. Is there a meaningful distinction between poor and working class, and between working class and middle class? A guy who works at McDonalds is working class, but so is the “assistant manager”. And the actual manager of that McDonalds isn’t exactly rich, and the owner probably isn’t either. It used to be that if you owned a business or were a professional you were middle class, if you worked at that business you were working class. But is the guy who owns a carpet cleaning business middle class? Just because he owns the vacuum and has a business licence, he’s suddenly leveled up? Is there a class difference between the guy who works at the car wash, and the guy who works at the car wash for twenty years and then buys a car wash? Because the reality is that many business owners don’t have a positive net worth…they took out a loan to buy that crappy business, and might take years and years to pay it all off, if ever.