Is Steem a Ponzi scheme? I can save you some suspense. The short answer is, "no." The same answer applies to Bitcoin, Litecoin, Monero, and pretty much every other reputable cryptocurrency out there
I suppose I should expand on that, though. I can say this with reasonable confidence because I know a little bit about how blockchains work, and I know what a Ponzi scheme actually is. This particular confidence trick is named after Charles Ponzi, an Italian-American con artist who created (or at least made famous) an investment scam that may or may not have begun as a legitimate business venture, but definitely ended with massive fraud. Somewhat like a pyramid scheme, revenue from each round of "investors" is used to pay off the prior "investors." A really successful scam appears to be a legitimate venture, and the first small groups of "investors" actually do profit as word gets around about this "sure thing" deal until there is a large pool of suckers and the con artist can make off with a load of cash by bailing at the opportune moment.
Crypto simply doesn't work like that. At least, most don't. I wouldn't be surprised if people can cite all kinds of shady blockchain shenanigans and scamcoins, but Steem simply doesn't fit the mold. You needn't invest a penny to start. I wrote and voted my way to dolphinhood without buying anything. Of course, it was as much a gamble as joining any other blog platform and altcoin network, but it's been going for over three years now. Prices rise and fall. Witnesses can adjust the algorithms behind the scenes. But there is no sign of a real scam here.
So, as your self-appointed Steem Pope, I advise skepticism whenever someone declares Steemit to be any sort of a scam. How does it fit the mold of a scam? Ask for an explanation. Odds are, it's someone who is really complaining about a poorly-timed crypto purchase, or who has been reprimanded by members of the community for abusive behavior. If there really is something fishy, it should be easily explained. And those who make false accusations earn a Steem Pope Excommunication.