That's an American point of view. Here in Britain there are pretty much only two main classes: aristocracy and dirty peasants. Doesn't matter what you do and how rich you are, if your ancestors didn't sit at the round table - you're a peasant.
*Upper working class (what the media likes calling the middle class)
It's useful, because it more accurately matches what the rest of the world means when they say "middle class". It's always weird watching British panel shows and the like when I hear someone refer to someone as being "so middle class" as a synonym for "so posh". Because here, it has basically the opposite meaning.
I'm from the UK and there are 3 classes. The Upper, middle, and lower. The 4th is not some bizarre difference between the landed gentry and the otherwise rich.
Upper class don't need to worry about money. You get some odd situations where they have it all tied up in some assets they don't want to sell but ultimately that's a choice.
Middle class can afford some luxuries. I had sailing as a hobby and classical music training as well as my own instrument to play, that's where a cultural element comes in. If the working class can't afford it, it's "posh". I didn't go to a private school requiring fees but it wasn't impossible financially if my parents made that choice.
Working class. Earn enough to just about survive. Cannot afford extras or luxuries. Hobbies are cheap. Gonto the local field and play football. The pub and play pool. Video games and TV. If music is your thing, it's the guitar because there's loads of those and skip the lessons, just learn off YouTube.
Unemployed and homeless fall below this. We've been developing a larger 4th underclass since 2010 and the dismantling of the social safety net. What was a tiny minority of the population has grown and may justifiably be called a class now.
But we each have a class reality and a cultural class.
I was born middle class. I experienced life as someone who was middle class. I financially dipped down into working class and experienced that as an adult.
So my cultural references and how people view my accent and habits are a bit of a mix. I'll appear posh to some and working class to others.
Some identify strongly with their class because ultimately it is about shared experience. You'll have someone who's managed to get rich still calling themselves working class. They might feel that way but the tragedy is other working class people they know will see the effect of joining the upper class. They'll get treated differently and eventually feel less working class. Those born into the upper class will still treat them differently because not having been brought up upper class, they have some working class mannerisms and references.
Hence the Yorkshireman sketch by Monty Python. Former working class men, now upper class. Discussing the old days.
TLDR: everyone really has 3 classes. The one they are in financially, the one they identify with personally, and the one others identify them as being in. Those 3 might match or might not.
Edit: as a bonus you can be a "class traitor". As an example Lord Alan Sugar grew up working class. Went from running market stalls to an electronics company to the UKs example of the apprentice. He was a Labour party supporter and the left leaning Labour party out him into the Lord's.
Even while a Lord some would call him working class because of his roots.
Then he flew back to the UK in order to vote through a right wing bill cutting benefits for working class people and the disabled. He's now a class traitor.
I don't think most people would restrict "middle class" to only those who can live off their investments.
We have a much more complicated relationship with class in the UK which is not well reflected by the language we use, that's certainly true. We often determine what class someone "is" by their social status and cultural interests moreso than the Marxist way. I read an article some time ago which identified 7 classes separated both by cultural and economic capital... This is closer to the reality in the UK imo.
I don't think that's true. There's definitely the three classes, but many people believe they're middle class when they're not.
It's basically impossible to become upper class. I think I read somewhere that it takes 3 or 4 successive generations at somewhere like Eton to be considered upper class.