Do you really think the billionaires are willing to have consumers so impoverished that they can’t continue to spend large sums of discretionary income buying the things that make the billionaires themselves richer?
You cannot just point at a system, say it’d be unsustainable and then assume nobody will let that happen.
Monarchies, lords, etc. have had much more reason to support their own countryfolk, yet many throughout history have not - has society changed enough that the billionaires have changed on this?
In summary: billionaires aren't as competent as you'd hope.