It doesn't have to be intended or designed as a Ponzi scheme to end up as one. E.g a legitimate investment manager could cover up a bad month/quarter/year hoping to fix it down the road then wind up being flumoxed by compounding losses or seduced by the increased business they get by beating the rest of the market during a downturn.
It seems like people just throw around the term "Ponzi scheme" without any understanding of what it actually means, saying that "Bitcoin is a Ponzi scheme" or "fiat currencies are a Ponzi scheme", when those statements simply do not make any sense.
A Ponzi scheme is when you offer an interest bearing or dividend paying investment, and you have to use the money obtained from new people buying into your investment to pay out the interest or dividends.
That's a very clear definition, and Bitcoin obviously does not fit - and it wouldn't fit the definition even if Bitcoin weren't based on a transparent protocol and open source implementations.
If I were designing a Ponzi scheme intending to defraud people, I wouldn't base it on a 100% transparent network of open source software using public domain crypto functions, where the wealth creation is by design not only randomly allocated, but mathematically capped to a static trickle rate.
It's like the most badly designed Ponzi scheme ever.