I don’t even get the point of them for billionaires. Like what, you’ll emerge after some calamity and say “well actually I was very wealthy and important in the old world”? These things are going to serve as glorified loot chest for the desperate who survive any sort of scenario that justifies their use. Assuming people don’t just seal up all the intakes and entrances to spite them. No one survives the surface? There’s going to be nothing for these people to return to anyway.
That’s all just assuming the security detail/staff transporting them don’t just shoot them and take their spot after hearing “thanks for getting us here and keeping us safe but there’s only supplies and room for my family, so back to the apocalypse you go”. So fucking stupid.