And at least in the US they have >45% of all the small arms weaponry. There is no bunker strong enough nor private army big enough if 100M people come for you.

They're probably be betting that the technology they will need to defend their bunkers, think autonomous kill-bots or whatever, will emerge before people start to riot.

Or they're planning to build an Elysium-like colony in the ocean or space, to keep the billionaire class far from danger.