Inequality has been written into the DNA of civilization ever since humans first settled down to farm the land. Throughout history, only massive, violent shocks that upended the established order proved powerful enough to flatten disparities in income and wealth. They appeared in four different guises: mass-mobilization warfare, violent and transformative revolutions, state collapse, and catastrophic epidemics.
I note that of these four, if warfare becomes nuclear warfare, the rest doesn't matter, and if not, it won't create a global shock; revolutions have been and will be quickly suppressed -- tech monitoring, armored tanks, ... ; medicine is stopping epidemics before they are catastrophic, and maybe we are now experiencing US state collapse.
My bet is on a plague or a global climate event.