It's impossible to become a billionaire after that without exploiting others, whether that is workers, employees, investors...whoever.
People say this, but I don't think it's true.
If I simply ask for people to give me money if they like me, and I get 1 million people to give me a dollar each, then I become a millionaire. Nobody's being taken advantage of, everyone is voluntarily doing this.
Getting to a billion is a lot harder but not impossible. If I ask and 10 million people give me $100 each over the course of 10 years, I might make a billion dollars that way.
So who can do this kind of "ask people for money" at these scales? Anyone who provides a service where the marginal cost of each additional recipient of that service doesn't cost anything. A musician playing music in a subway station performs basically the same amount of work whether 10 people walk by or 1000 people walk by in the time that he performs. And if you're a recording artist, you might release a song that literally over a billion people enjoy.
Yes, sports leagues and movie studios and record labels and Ticketmaster and book publishers and live venues and broadcasters and tech platforms are often exploitative in many ways, but authors, musicians, artists, filmmakers, comedians, and other creators can and do sometimes do things that make the world better by billions of dollars worth of happiness, while taking a cut worth hundreds of millions, or even billions.
Ultimately, we do things that produce value in some way or another. Sometimes we get to keep the fruits of our labor, and sometimes we get to profit from that value created. Often, as in the world of intellectual property, the value is very far removed from the actual cost to produce, including the cost in terms of human labor. When that happens, sometimes the excess value is worth billions. Even without a big team creating that value.
No, that study was debunked. Turns out that some subset of unhappy people will remain unhappy even if you give them all the money in the world, so looking purely at the least happy people in America, you'll notice that their happiness stops going up at $75,000 in 2010 dollars. But if you focus on people who are already on the happier side of the spectrum, more money keeps buying them more happiness, even if the slope of that relationship tapers a bit.
Side note, the way the two sides reconciled their methodologies, that produced different results, was a really interesting way to perform science, and should be followed in the future whenever there are well respected studies that contradict each other.