Scenario A: Someone breaks a window and the homeowner buys a replacement.

Scenario B: A homeowner adds a new window to their home.

Scenario C: A homeowner buys an online-course to learn how to make windows and then adds one to their home.

Scenario A has approximately no benefit to the economy. The homeowner is no better off (same number of windows) but had to spend money. The window maker might be better off, but only to the same extent that the homeowner is worse off.

I totally agree that Scenario A is not a benefit to the economy. That's the "broken window fallacy".

But Scenario B is definitely better for the economy. The homeowner has decided that having a new window is better than having the money. So the homeowner is better off. The window maker is also better off because they get the money. This is what happens when a billionaire buys a yacht.

Scenario C is the best. The homeowner has a new skill, which they can use to add more windows to their house or maybe their neighbors' houses. Over time, the amount of money spent on window-making will decrease, but the number of windows will stay constant or increase. That's a net benefit. And the online-course creator still made money.

This is what Musk is doing. He is developing new technologies that enable new capabilities and/or make existing things cheaper (e.g., electric cars, access to space, rural internet connectivity).

There is also Scenario D: The homeowner doesn't buy a new window but just keeps his money under his mattress. This is clearly the worst for the economy. Hording money like that means that there is less money circulating and lowered economic activity. The window maker is worse off, and even the homeowner is worse off if they would like to have a new window.

Billionaires who don't spend their money are the real danger, not the ones who tweet too much.

Investing their money is slightly better in that it makes the price of borrowing cheaper. But that only helps up to a point. Someone has to spend money or else there's no point in being able to borrow some. So I wish more billionaires were following Scenario C.

In your original comment, it sounded like you would encourage billionaires to buy yachts, solely "for the good of the economy".

Scenario D: A homeowner adds 10 window to their home because the populous think he is stingy and will send him to the guillotine if he does not start spending his money on new windows!

Scenario D provides no benefit to society.

If the billionaire does want the yacht, then no encouragement is needed.