If you want to get into semantics and argue about the definition of a ponzi scheme, then you are missing the point entirely. At it's rawest, the re-pricing of assets given a finite and fixed size system (ie, all the money is retained in the system), is a function to transfer wealth. In this situation, the zero-sum game means that one person's gain, is passed along to someone else's loss (it could be many people). Now this is the simplest scenario where we assume a closed system.

However, for an open and expanded system, it's really just a matter of passing the hot potato on. The ever need to expand the user base is fundamentally about bringing more money into the system as a whole, otherwise the equilibrium of price of steem will fall due to an expansion of steem / sbd being issued whilst a non comparative amount of it's price paired asset is coming into the system. In effect, the earlier holders of STEEM explicitly require that the system expands in order for them to see a marginal increase in price of their investment, without which, it would just be a matter of redistributing wealth between existing stakeholders.

In summary, a zero-sum redistribution of wealth is what happens when the system doesn't expand. When the system expands, in the case off steem, it has to do so at a faster pace than the issuance of steem in order to sustain price appreciation. This in practice means more people bringing fresh money from outside the system, into the system and buying from existing holders.

If this were the ONLY purpose of STEEM then it would be a ponzi scheme, but thankfully there is utility and value in attention which is the main fabric of Steem and why it sets it apart from being a ponzi.