I think most billionaires don't realize that when shit hits the fan the civilizational rules and support structures that made them rich and powerful probably will no longer be there for them. They will be operation in an entirely different environment and essentially starting over. They will have to negotiate, fight and die for their lives just like everybody else so they had better provision for strongly motivated protection. No matter how rich they are and stratospherically superior to the useless eaters now, the end of civilization will bring almost everyone down to the same level as new warlords rise up and take what they can from everyone else.
Can you be a tech titan when there's no longer any tech? No.
I think most billionaires are either not too bright or have never heard of Quikcrete. Every underground bunker has to have pipe going to the surface to draw in fresh air. Find those air inlets, fill with Quikcrete, and watch how fast the billionaires open those very expensive but worthless blast proof doors.