I think (to me) it’s about background.
I consider myself working class because I grew up on social benefits, with food banks and what else with two parents who didn’t even graduate high school, and I was the first (and only) of my siblings to actually graduate high school (I am born in the 90’s and most of my siblings in early 2000 so definitely not at a time where people didn’t make it to high school) and both my parents work low paid jobs that come with not much recognition.
I made it to university but quit after 2 years and ironically started a career in a similar area to my mom’s except a niche part, and now I am quite successful at it which means that I now do earn a very good salary (6 figure range), travel a lot (while we never went on holiday further than 2 hours away as kids), own my own home that is paid off and overall don’t have to think or look at finances too much (though I am massively careful with it due to how I grew up.).
So technically I live a middle (?) class lifestyle currently but I do NOT consider myself middle class because I strongly come from a working class background and feel I lack a lot of culture and knowledge that I think comes with a middle class background and a lot of how I approach life, finances, politics and the world is rooted in my background and definitely stems from being working class? If I had kids though I think they would be considered middle class because of the background they would be born in, which isn’t technically working class anymore.
Personally my mom comes from a middle class background, but she made decisions very young that now makes her fall strongly into working class, her sisters made completely different choices and they are now both middle/upper class, and you can see the massive difference it has made on all aspects and also how my cousins have come up (they all are strongly middle class) and there is definitely a massive cultural gap within the family and I do know my aunts and my mom’s family at large rarely struggle to relate to us. And for example my cousins have not only had access to more money than us but also access to more knowledge and skill sets (such as money management tips and advices from their parents etc…) that has given them a real leg up and also make them most likely to stay middle/upper class.
So I do think background is the main element. And how much the cultural inheritance that comes with being middle class plays a part in the someone’s likelihood to go further in life either socially, financially or career/study wise.