An apocalypse may have a slow onset. Not: everything normal one day, hungry / violent mobs in the streets the next day.
Services, supply chains, store shelves etc gradually degrading / emptying over the course of weeks, months or even years.
Also: a collapse of society could be limited in scope or duration. Social unrest, widespread protests, looting & other crimes, but things returning to 'normal' within months. Those security guards may consider how much they want to keep their jobs after such an event.
In short: societal collapse comes in many shapes & sizes. Private bunkers etc are just nice-to-have options for the ultra-rich.
Good considerations, a couple of points in response:
1. Do you know anybody who has gone through a revolution? E.G. I know lots of east-block programmers who lived through the fall of the iron curtain. If so, ask them this question:
"How long, before the revolution actually happened, did you know it was going to happen?"
Uniformly, everybody I ask always says they had no idea until the day it happened. Sure, these kinds of things don't happen overnight, and in retrospect, it is often easy to see how things were going to inevitably lead to it.
But apparently its really hard to see these tells before it happens.
2. But suppose for sake of discussion that some billionaire was observant enough to see it coming in time for him to arrange transportation to his castle.
As long as there is an external legal system, a rouge guard could be tried for theft or murder and locked away. But should the legal system collapse, the guard finds himself in the position that he is the one with the guns, and that whether he steals from or kills the owner of the castle is entirely up to his own decision.
As the external society gets worse and worse, and as the guard is observing that the billionaire refugee is living in comparative luxury, and that the refugee can only pay him from the very store he is guarding....
...why would he do all the work, and take all the risks of being a human target, while somebody else gets the lion's share of the benefits? When he could just turn the gun on the refugee and kick him to the curb and have it all?
Services, supply chains, store shelves etc gradually degrading / emptying over the course of weeks, months or even years.
Also: a collapse of society could be limited in scope or duration. Social unrest, widespread protests, looting & other crimes, but things returning to 'normal' within months. Those security guards may consider how much they want to keep their jobs after such an event.
In short: societal collapse comes in many shapes & sizes. Private bunkers etc are just nice-to-have options for the ultra-rich.