There's not one single person in the world who should own a thousand million dollars, never mind hundreds of thousands of millions of dollars.
The pure existence of billionaires is unethical and immoral - doesn't matter whether they're being stupid and fascist in public, or quietly pulling strings and bending society to their will in the background.
These are basically small concrete boxes sunk into the ground. They're only meant to stick out a bushfire for a few hours.
You could probably just keep a few bottles of oxygen or a carbon dioxide scrubber stashed in there, just in case. If you can spend $10,000 on one of these bunkers, spending a few hundred more isn't going to make a difference.
Anything longer than a few hours would get dicey anyway without room to move around, without room to stash water or food, without a toilet or beds.