Sure, but you can say the same about anything. Take a company's stock, for example. There are 1,000 shares. Early on the trade at $5/share. Later they trade at a higher amount - say $500/share. The people that bought it at $500/share can only make money so long as other people are willing to give them that amount for it. If they can't find a buyer, they lose money. Yet nobody would consider this a Ponzi scheme, I hope.
It's just disingenuous to call it a Ponzi scheme. "Ponzi scheme" is being used not as an accurate descriptor, but as a not-quite-accurate term that has negative connotations. It's just FUD, more or less. Call it what it is - a protocol and a public ledger where people are currently willing to pay money to post a transaction to their account, and where people might not be willing to do so later. Not quite as bad when you put it that way!