I'm reminded of that line from Chinatown. "Why are you doing it? How much better can you eat? What can you buy that you can't already afford?" Is there really no point of diminishing returns, where you can tell yourself you've made enough money?
My fantasy about money is not about better cars or nicer houses. I'd like to do what Elon Musk is doing - try to advance human civilization through projects like space exploration. I'd be launching space telescopes, more probes to outer planets, etc.
The American investment banker was at the pier of a small coastal Mexican village when a small boat with just one fisherman docked.
Inside the small boat were several large yellow fin tuna. The American complimented the Mexican on the quality of his fish and asked how long it took to catch them.
The Mexican replied, “Only a little while.”
The American then asked, “Why didn’t you stay out longer and catch more fish?”
The Mexican said, “With this I have more than enough to support my family’s needs.”
The American then asked, “But what do you do with the rest of your time?”
The Mexican fisherman said, “I sleep late, fish a little, play with my children, take siesta with my wife, Maria, stroll into the village each evening where I sip wine and play guitar with my amigos, I have a full and busy life.”
The American scoffed, “I am a Harvard MBA and could help you. You should spend more time fishing; and with the proceeds, buy a bigger boat: With the proceeds from the bigger boat you could buy several boats. Eventually you would have a fleet of fishing boats. Instead of selling your catch to a middleman you would sell directly to the processor; eventually opening your own cannery. You would control the product, processing and distribution. You would need to leave this small coastal fishing village and move to Mexico City, then Los Angeles and eventually New York where you will run your ever-expanding enterprise.”
The Mexican fisherman asked, “But, how long will this all take?”
To which the American replied, “15 to 20 years.”
“But what then?” asked the Mexican.
The American laughed and said that’s the best part. “When the time is right you would announce an IPO and sell your company stock to the public and become very rich, you would make millions.”
“Millions?…Then what?”
The American said, “Then you would retire. Move to a small coastal fishing village where you would sleep late, fish a little, play with your kids, take siesta with your wife, stroll to the village in the evenings where you could sip wine and play your guitar with your amigos."
(Of course, the MBA glosses over scaling the catching of fish, as though all fish have the same qualities.)
The easy way to productively spend $X billion is to invest it in space/physics/medical research or development. If you need to blow a few hundred billion, you could launch an asteroid mining venture with a reasonable chance of turning that money into a few trillion and making space development vastly more affordable for the rest of humanity.
Q: How do you make $1 million in aerospace?
A: Start with $1 billion
EDIT: And of course, the response to that line in the movie is "The future, Mr. Gittes."
In your example, it seems like you are saying "there is more happiness in mining asteroids than in being able to live very comfortably, indefinitely, without having to work to support yourself".
Except for a very narrow portion of humanity, this seems implausible. Would mining asteroids really bring fulfillment to a person who has experienced ennui when climbing a mountain, scuba diving the barrier reef, yachting the Mediterranean, or designing their own home?
Failure of imagination, sure, but I think you're going to be disappointed if you think that a billion is going to be deeply fulfilling relative to 20 million.