I mean, does having $90B confer any benefits above what having, say, $20B does?
Of course, that difference matters to companies; the more money you have, the easier it is to buy or destroy competitors.
But for individuals, am I missing something?
Musk has decided to allocate resources to sustainable technology and space exploration. Gates has allocated resources to the eradication of disease. Roman Abramovich has allocated resources to sports teams and a series of increasingly massive yachts. $10bn might get you a football team or a big boat, but it won't get you to Mars.
It's not necessarily fair or just, but it seems to be quite effective. Lots of alternative systems have been tried, but they've all been woefully inefficient at directing resources towards activities that generate more resources. Capitalism is the best system we've found for coupling "stuff-making ability" to the amount of stuff you get put in charge of.
Capitalism doesn't reward hard work, which feels very unfair; there's no real relationship between how hard you work and how much value you create. The typewriter industry was full of incredibly talented and hard-working typewriter designers, but the value of the best typewriter is far less than the value of a crappy computer.
Capitalism is also really bad at distinguishing between fairly created value and value that has been captured by theft, fraud or coercion. For that, you need an effective legal system. Free markets and bad laws create a lot of rich gangsters. People who control a lot of resources can use those resources to influence the legal system, which is inherently dangerous unless you've got really good checks and balances.
Just Google what rich people say about being rich and you'll find that they say exactly the same thing. After a certain point it's all the same.
Anyway, if you have the personality that creates a company like Amazon, you can't just sit back and enjoy it. It's just not in your nature. I had an uncle like that - he kept starting companies long after it mattered financially. He'd sell the current one, tell everyone he was retiring, sit around miserably for a few months and then start another. "It's mostly just a hobby," he'd say, and sell it for ten million dollars a few years later and start the whole process over.
Also, flooding your descendents with money isn't necessarily a good idea. People who grew up with more money than they could ever spend tend to be pretty unhappy. Warren Buffett claims he left his children enough money to do "anything they want to do, but not enough to do nothing." The rest he gave away.
Like energy, wealth can not be destroyed - only transferred.
The reason billionaires are allowed to exist is because they do a good job of maintaining that money for themselves and others.