I think the heart of the matter is, what is the nature of your stake/holdings within your own country? Do you have a form of power and agency that the political machinery must respect but does not revere? The numbers aren't what matters and would necessarily vary wildly across the nation anyway. What matters is how vulnerable you are to the effects of wealth inequality. That vulnerability is what should be getting highlighted, and I think it captures what was on people's minds back in the 90s as they talked about the middle class shrinking. It was not just wealth but power concentrating either on you or (far more likely) away from you.

In capitalism, capital is the only real power and politics only a moderating force. So the health of a democracy can be measured by the distribution of power -- i.e. the size of the middle class. While you can live a good life in the lower class (which may be inescapable due to such things as being disabled), it's by the grace of whomever holds power over you or the social systems that a majority (hopefully) dictates shall respect persons rather than property and ability. The lower class has no intrinsic power, so when middle class falls below majority

Below middle class, you are disenfranchised even if you still have a vote, because the economy sees you only as a burden and markets have no natural incentive to consider you. If you are lower class and the system hasn't the grace to protect your interests and quality of life, that is the system's failure. Above middle class you are privileged with the capacity to force economic changes others do not want, and on top of having a vote the political system will defer to you wherever you hold the capacity to help or harm -- and ignore the voting power of your lesser opponents. That's to say nothing of your ability to influence lower and even some middle class people to vote your interests instead of their own. If you are upper class and the system hasn't the fortitude to both constrain and redistribute your power, that is the systemic failure that most erodes middle class power. Once middle class no longer holds the majority of power, capitalism spirals into "late stage" and steadily grows the lower class while undermining their supports.

Middle class is where fair equity lies, by virtue of resilience against the abuses of wealth inequality. And yet though it should be as big as possible, middle class is the only one that can definitely be entirely empty.