Billionaire bunkers are just billionaire tombs - the only reason billionaires are in charge is the very society they plan to hide from in their bunkers. They have plans for bomb collars, but someone has to maintain them and they could deactivate them, and if your bodyguards want you dead then you're dead.
It doesn't have to be literal bunkers. Just look at the structure of your average "third world" city -- glass towers and luxury condos ringed by miles of shanty towns and slums -- and then a barrier and villas and luxury estates out in the countryside.
This is, in fact, how the end of the western Roman empire looked. No bang, just a long series of whimpers as the elite retreated to armed estates with private armies, taxes stopped being collected, overall trade declined, cities declined, and the agrarian peasantry became bound to lords and estates out of need of protection, etc. etc.
When civic institutions and the taxes that support them are openly attacked and predated upon, weakened and then combined with environmental, disease, or security crisis, one needn't think hard to picture the kind of eventual world that results.
