Who is more upper class, a relatively impoverished Prince of the Holy Roman Empire or a billionaire by marriage and luck? Managers like the mentioned bank CEO, or someone who can rent them a house in the Hamptons because they have lived there for generations? An intellectual who has never really worked, or a wealthy rapper who didn't finish high school?
I think Western society has been undergoing a transition between two different class systems. (Marxists would surely connect this with the economic transitions from feudalism to mercantilism to early capitalism to late capitalism.)
In the mediaeval class system, class was determined by social status which was predominantly inherited, and money alone was not enough to move from the bottom to the top. A person born poor might acquire great wealth, but they would still be locked out of the nobility/aristocracy, unless the monarch deigned to ennoble them. An impoverished baron socially outranked the far wealthier merchant who was born into poverty and worked/lucked their way out of it.
In the late capitalist class system–all that really matters is your net worth. Everything else is secondary. Who cares who your parents were or where you come from once you have a billion dollars to your name – and if a billionaire is excluded socially, it is very likely due to something about their individual behaviour (see e.g Kanye–although I suppose he's an ex-billionaire now), rather than their family background in itself.
The transition between the two systems has been ongoing; it still hasn't completely finished, but we've moved a lot closer to the late capitalist system, and a lot further away from the mediaeval system, than we were 50 or 100 years ago. I don't think the transition proceeds at the same rate in every country either – I think the UK retains more of the old class system than most other places (although even in the UK it is a lot weaker than it used to be); it also arguably retains some strength in the northeastern US, although not to the same extent as the UK.
My point is that different people look up (or down) to different people by different criteria: "net worth" interests competitive capitalist adventurers, "who your parents were or where you come from" remains a high priority when someone is wealthy enough for their lifestyle.
I certainly find it plausible that if A and B are of roughly similar wealth, but A was born into wealth whereas B lucked/worked/married into it, A might see that as a reason to look down on B.
However, I suspect if A was born into $100 million and B lucked/worked/etc into $100 billion, then all else being equal, I doubt A would look down on B in the same way. I expect they'd more likely view B with envy, as a potential business opportunity for themselves, as a friendship which might give them access to things they themselves can't afford, than as a social inferior. When the gap in wealth becomes big enough, it tends to drown out all the other factors.
The huge class divides are not between retail workers, tradesmen and software developers, the divide is between C-suite executives / old money, and... everyone else who has to work for a living.
As a tech worker, I have far more in common lifestyle-wise with a schoolteacher than I do with the SVP of Engineering in my own company.
In the US, working class tends to mean people in food service, retail sales, low-skill manual labor, people relying on gig work, and some very low-level white-collar workers like secretaries. Upper class are basically the very few people for whom work is optional. F500 CEOs, SVPs, billionaire investors, and so on. Their existing wealth grows at a rate that can sustain their and their family's standard of living indefinitely.
The rest is middle class, a very wide range from schoolteachers, to skilled tradesmen, to engineers, to university professors, to small business owners, to doctors and so on.
My point is that people are nit-picking when they say things like "Oh, but doctors are upper middle class and teachers are lower middle class and engineers are lower upper middle class and on and on and on. Distinction without a meaningful difference.
I think traditional concepts of social class count for a lot less in the UK of 2023 than they did in the UK of 1973. But wealth counts for just as much, even more.
Were Rishi Sunak's parents "upper class"? And yet, he'd surely be a far more valuable social connection to have than Baron Forgettable who happens to be the King's fifth cousin. And I think that will remain true even when his prime ministership is over.
Class can be more fine-grained than just lower/working-middle-upper
People commonly talk about “upper middle class” vs “lower middle class”, and then I suppose there are even some people in between the two (middle middle class?)
Well, in the same way there is surely lower upper class and upper upper class, and maybe even more levels than that. A person on an annual salary of US$25 million is undeniably upper class but still far beneath the centibillionaire class.
There’s also no clear boundary between upper middle and lower upper. You may think someone on US$250K is “upper class” but they probably only think of themselves as “upper middle class”, and personally I would think the same. In my own head, to really be upper class, your annual income has to be (consistently) measured in millions, not hundreds of thousands.