Your mind produced a wonderful list of rabbitholes there! Lovely job.

I don't feel this is a great response tbh. While you're comparing money, you're not differentiating how government money is different than corporate money, which are both different from personal money. Even if a person wants to build a tunnel of that magnitude, absolutely no way are they going to do it out of their pocket, at worst it would be through corporate funds. Its not like these guys are spending their own money on those things you're talking about. Even Elon still gets others to invest in his companies.

An important thing to note is that it takes time to spend money and money makes money. Even a very modest interest rate yields an incredible amount of money. The money is so large, it works differently than money does for us normal people. For example, say you have $30bn a year and are getting a modest 5% interest yearly, that's: $1.5bn a year, $4.11m/day, $171.2k/hr, or $2854/min. As you can imagine, this has a big effect. If you're trying to buy a McLaren F1, well, you can wait 5 days. Which it'll probably take that long to make this transaction happen anyways. So there would be no net loss of money. So, supposing you wanted to make your $15bn (I know, but we're approximating anyways and they're similar anyways) tunnel, you somehow got permission to do it, and you are doing it all out of your pocket, you have to realize that this does not equate to taking out $15bn from your net worth. Because... it takes time to build such a tunnel. Let's approximate again, saying it takes 5 years (very aggressive) and you pay at the end of the year, then the net decrease of your wealth is $7.5bn, not $15bn. At 10 years, you have no net change, and any longer you've had no effective loss. (To be clear, I'm not saying there's no cost, I'm saying that it doesn't change your net worth)

The problem is, your money makes money too fast. For most things, faster than you can spend it. Every second you aren't spending it, it grows.

I should also clarify an assumption that I guess is not obvious. I'm saying it is essentially impossible to spend these amounts of money on things that personally benefit you. You can't buy enough houses, personal jets, yachts, and so on. We agree. But mega projects take a lot of time, as well as philanthropic endeavors. If you want a real world example, see MacKenzie Scott. She has given away 50% of her wealth, yet her net worth is identical to that in 2020, when Forbes started tracking her (divorce was in 2019). She is worth $36bn and has given over $17bn away, and in those 4 years (I'm sure some of this money is pledged and not yet in peoples' hands but that's standard philanthropy, right?). So yeah, she could have built that bridge for no change in her net worth. ¯\_(ツ)_/¯

But in your favor, I guess Elon did buy Twitter and with his great negotiating skills offered a 27% premium (and not in a one-time lump sum). But also, this has seemed to have no real effect on his net worth due to how his wealth is actually held and showing the nativity of our approximations, as most billionaires are getting much more than 5% interest on their money.