That, at present, is where the wealth is coming from.

I would argue that increasingly it is not. The relative value of labor is and has been declining due to automation.

Money is their only real leverage.

It isn’t - there is also legal ownership, of natural resources and other types of property, and there is the force backing that ownership, which is also subject to automation.

If you are skeptical about the idea that wealth can exist at all independently from labor, consider the distinction between a dictatorship with an economy based on oil or mining and a more democratic country with an economy based on a diverse array of skilled professionals. Yes, in both cases laborers are involved in what the country produces, but in the latter, circumstances give them more leverage, because their active engagement and relative consent is more of a prerequisite to achieving that product. That leverage equates to a higher market value of their labor. I can imagine a future where everyone is effectively reduced first to slaves in a mine and then to skeletons next to mining robots.

wealth that can be extracted from the working class

This is my point though; they aren’t going to need to do that.

most of the lower class won’t be able to purchase anything other than essentials like food. No new cars, no tech gadgets, no fancy dinners, no vacations, no disposable income.

Bold of you to assume the rock bottom of wealth inequality includes the ability to purchase food and is survivable.

When we get there the economy collapses because there’s no money going into it. The profits stop rolling in, unnecessary goods stop being produced, and the luxury goods producer’s shut down. At this point the money they worked so hard to hoard becomes worthless because they can’t buy anything with it.

Money doesn’t come from people, it comes from the fed issuing debt. The economic “value” backing that money also doesn’t necessarily come from people, it comes from control over things that are valued, which may include human labor, but that labor can be automated. The actual value of human life is not represented by money or other financial instruments.

Economic constraints aren’t preventing the world from decaying into an enormous desolate golf course.