Not really. It's slightly more complicated than that. Often, when "stars" get rich, they no longer need to work. Or, they choose to change their "entertainment" in a direction that the agent does not want.

Katy Perry, Selena Gomez, Britney Spears, etc, could have just bought an Island, and lived out their days on Royalties, and proper investments. But, the agent is there to keep them in the machine, as long as is needed(which is often longer than the performer desires).

Convincing a person who has a net-worth over 50 million dollars to travel the world, and work VERY HARD every day of their life isn't exactly easy.

Also, there's another problem. These musicians/entertainers wield a MASSIVE amount of power. People listen to entertainers more than politicians, religious leaders, and even their families. That's why they're so heavily controlled. And that's why such extensive effort is taken to make sure they don't make $100 million dollars off the machine, then start destroying it. Michael Jackson is an example of this, and they had to frame him for child abuse, and eventually kill him, to shut him up, because he was too powerful.