If someone claims Bitcoin is a Ponzi scheme and you want to address that claim head on and honestly, what you have to argue is that there's a reasonable expectation that Bitcoins will appreciate in value over the long term due to demand for Bitcoins as a means of exchange. If you don't believe that, then Bitcoins as an investment basically *is* a Ponzi scheme.
Bitcoin is more like investing in a patent. The invention of the bitcoin is specifically intended to drive the economy of the internet. There are many other alternate ideas that are for sale, but Bitcoin is the original. It is better than a patent, because it cannot be counterfeited. It is not like buying stock in the company that owns a patent, but rather partial ownership in the entire production chain. Expecting a profit on the investment is not based on hearsay, it is opened sourced and based on the research of some of the greatest minds in math and cryptography. I can understand if you are trying to sell someone on the idea of investing in bitcoin on your say-so, but you don't need to. All the code, whitepapers, and wikis are available for anyone to research.
Perhaps some people interpret terms like Ponzi scheme to mean a risky investment, but I perceive that it's more like investing in a fundamental technology itself. This idea is too difficult for most people to wrap their mind around so they reach for the easiest analogy. Bitcoin requires not only people willing to monetize the tokens, but also requires a form of cooperation that forms every social relationship. Without getting into the woo zone, Bitcoin is more like the discovery of atomic energy and Bitcoin is the Manhattan Project. Sure, you could build your own, but not before Bitcoin is adopted by the majority of socially adjusted people that understand the purpose and usefulness of this tool to benefit the majority.
Normally, paradigm-shifting ideas like this are never fully understood by the masses, but instead they trust the peer reviewed experts that endorse the technology. Bitcoin is not a political tool. Bitcoin is not merely a financial tool. Bitcoin is a social tool for communicating value on a global scale. It can be used for much more and has the potential for becoming a powerful social science tool (because of its anonymity), but for now it can be developed to scale and help humanity break through many of the societal barriers created by nations and globalist powers. Someday Bitcoin will even replace these barriers.
tl;dr Bitcoin is too hard to wrap your head around, like nuclear physics. People use the term Ponzi to describe anything financial they don't understand. Bitcoin doesn't need salesmen, it needs trusted scientists to approve of the technology before it will be adopted by the masses.