How do you co-opt a definition that historically includes you?
If you're in the top 10% of wealth, you can't claim to be "middle class" in good faith. It requires co-opting a term that applies to the rung below you.
If the "working class" are blue-collar hand-to-mouth labourers and the "upper class" are the wealthiest who hold the most socio-political strength and the means of production, if you are far closer to the upper class than to the working class, the best you can claim is that you are "upper-middle class", but I can tell you for certain that people in that bracket consider themselves to be purely middle class erroneously.
If anything, the "aspirational" aspect of the term means that it has been the "working class" that has "co-opted" the definition as first world wages rose from the 50s to the 80s/early 90s to the point that by the 80s through to the present the "working class" began viewing themselves as "middle class" when that term never really applied to them.
I mean, if teachers, police officers, postal carriers, and entry-to-mid level engineers aren't "middle class", what are they?
My father was a truck driver. ("was" because he got injured and had to change jobs, he's not dead). Surely truck driver is a "working class" career. But wait! In 2003, as a transport truck driver, my dad pulled 70k a year. In 2017 dollars, that's 88k! Holy shit! Not only was he clearly not "working class", he clearly wasn't even "Middle Class"!
My father, hauling cars between auto plants to car dealerships all between Detroit to the GTA, was a member of the upper class! Who knew?!
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All of this is just to say this: "Class" perceptions are NOT just about income. They weren't historically, and tying them strictly to income levels gets REALLY messy if you are committed to it.
You know why your father makes that money and counts as middle-class?
a) He didn't start off that way and got to where he was through at least a decade of wage increases
b) the class system was redefined with the generations that came after him by the changes in cost of living
I can guarantee to you that a 20-something doing the same job will never achieve the financial security he enjoyed. EVER. That job is more working-class than ever. And it's nothing to do with his wage being higher, it has to do with his wage going a lot further than it does now due to house pricing increases,
post-secondary tuition increases of 200% (and let's not kid ourselves, PS-educated people are in working class jobs now more than ever in history), a rise in the costs of auto ownership, etc.
Jobs that were typically defined as "middle class" no longer afford the luxury of things that the middle class were able to achieve, like strong retirement savings, any degree of financial investments and a debt load that wouldn't outlive them. This is due to both declining earning potential and a cost of living difference.
So there are 2 options and both of them require acknowledging that the term "middle class" has changed:
1) We acknowledge that the middle class of 21st century are working the same jobs as those of the 20th century middle class but with a greatly reduced economic prosperity, or
2) We acknowledge that the "middle class" is still a term based solely on quality of life that prior "middle class" jobs don't facilitate anymore and therefore the middle class is such a small segment of the population that it's effectively disappearing and not nearly as big as politicians who "champion the middle class" or economists who use the "middle class" to measure economic prosperity are comfortable admitting to.
In either case, using the term without establishing which definition you mean to let the rest of the population know if their feelings about their place in the class system align with that politician or pundit's definition is irresponsible at best.
If we're going to use the term, it needs a firm definition under a modern context by every person who uses it or it ends up being a meaningless buzzword. If it didn't need personal definition, we wouldn't have politicians or even ourselves using the term to mean 2 different things and confusing the issue by talking past one another.