I've been thinking that you can divide businesses on two axes,

                            Scalable - Many customers
                                     |
    Short-term/       Ponzi Scheme   |    Monopoly         Long-term/
    Transactional  --------------------------------------   Relational
                       Contracting / |   Consulting /
                       Retail etc    |   Therapy etc
                                     |
                        Non-scalable - Few customers

And mathematically, only businesses at the top of the graph are capable of generating a billion dollars. Hence, if you are looking to be a billionaire, the path lies either through a Ponzi scheme or through a monopoly. Both of them, in their most pure form, are illegal, and the challenge in the business model is to execute on them while staying just barely on the right side of the law.