That's a bit of an extreme made up example, which wouldn't be very representative of the common case, but I'll entertain it. Just keep in mind what's true of an extreme edge case, is not necessarily true of the common case, and you can't extrapolate like that.
Say there was a 100 billion yacht, it still served as a means to redistribute flow of funds to others. Those engineers, material extractors, workers, staff, all that contributed to people being given some of that money.
This goes towards creating an economy, and good economies can sustain countries and lift many people out of poverty, saving many lives.
Creating a demand for people's labor and resources helps with creating an economy.
This isn't money wasted, the money went to many people, it was a way to redistribute the money to others.
So I don't see the harm at all, at least not from a financial standpoint, you could argue about environmental harm though.
Now what about opportunity cost? Sure, maybe that 100 billion could have been used in ways that redistribute it more evenly, more fairly, or to people who needs the money most, or towards an endeavor that has longer term uplifting outcomes, etc, there might be a better use, though it becomes hard to figure out what use is truly better.
The way I see it, there's two dimensions:
- Redistributing money, who gets it? Split amongst how many people?
- Potential for future returns, is the money spent in a way that gives people the means to produce more?
The yacht definitely redistributes money, but maybe you disagree about the distribution of it, who got the money.
Arguably the yacht could bring future returns. Creating a demand for engineers pushed more people into engineering, now more people have the engineering know how, they go on to use that to build other projects, creating an infrastructure that others can then leverage to make other things more efficient and build on top, etc.
Basically, I just don't think it's as black and white as you claim, even in this extreme example.