The way I see it: the underclass has to work hard to move upwards, the middle class has to work hard to stay where they are, and the upper class has to work hard to move downwards.
Not to get too soap boxy, but that's exactly how the upperclasses have used divide and conquer to hold onto disproportional political power. It seems "wrong" to you, but imagine we went back to the tax rates set in the Revenue Act of 1942 with brackets adjusted for inflation: a family with a single tech exec or engineer making 6 or 7 figures a year can live well within their means and still have to drastically cut back on their quality of life under those tax rates. Someone with a hundred million dollars of productive assets probably wouldn't notice a change in QoL at all, unless they measure it by yacht-foot or jet-seat.
Almost all of us are N paychecks away from bankruptcy. The only difference between us is what number N is.
There are people who do nothing and dividends from their assets pay for their lifestyle and grow their net worth forever without labor. Those are the few actual upper (economic) class people!
I think it's still middle class at best if the assets have to be sold to realize the gains. Full upperclass (IMO) means the ability to leverage assets to acquire even more assets (especially in down markets) without risk of catastrophic loss while funding a lavish lifestyle.
It's an important distinction because short of total economic or societal collapse, productive assets held as collateral continue to generate capital. One can raise large amounts of capital almost on demand with extremely favorable terms using those assets, essentially multiplying the wealth that the upperclass has at its disposal. Once they hit a certain scale, it's almost trivially easy to make modest but consistent returns on borrowed capital because investing is a lot like farming: being a farmer and owning a single farm is really risky due to unpredictable stuff like droughts, but being Tyson and owning farms all over the world is really low risk unless the entire planet is hit by a drought simultaneously, in which case everyone will get bailed out.
Imagine you own a hundred million dollars worth of Blockbuster franchises free and clear that generates a "paltry" $2 mil a year in profit. What do you do when your investment is threatened and Netflix IPOs at around a dollar in 2002? You go to the bank and get a $25 million loan to buy Netflix stock. Then in 2008 when it's at $5/share and the stores are barely generating enough money to cover the loan payments, you firesale the remaining Blockbuster assets to pay off the rest of the loan and laugh yourself to the bank after selling $125 million worth of Netflix stock for an annualized return of around 3% in a recession. That's a very contrived example but it illustrates the point: with productive assets, you can use the proceeds to pay the cost of hedging those assets. When you do that across a large, diverse portfolio, you have to try not to make money.