Money, property, limited companies, intellectual property laws, patents, contract law, etc. etc. are things on which all successful businesses depend. These are social constructs - societal "technologies" if you like. They work because as a society we agree that they should and enforce them through the rule of law.

The assertion that it should be "impossible" to be a billionaire (or trillionaire, gazillionaire, whatever) is really an assertion that a just and moral society would design all of these things to prevent that outcome.

And I think it's pretty reasonable to say that we ought to set society up such that as someone gets wealthier we take money away from them at faster rates, so that beyond some level of wealth it is very difficult to continue to get richer.

Unfortunately, a lot of people are captured by rather libertarian ideas about government, money, property, etc. that seem to prevent many people (at least in the US, UK, et al.) from behaving in anything but the most selfish, individualistic, and antisocial of ways.