Companies usually have a business model. They produce something that a target market will pay for.
Steem's only source of money flow is the investors themselves, and the invested money is distributed between content creators (and other rewrad earners) cashing out, the founders and early birds cashing out, and yet the investors expect to get at least as much money out of it as they invested (otherwise, what's the point of investing?).
So more money is flowing out of the system than into the system, unless they balance this by constantly increasing the number of new investors.
Unless of course, investors are not in it for the returns. They accept that their investment will be distributed and they will actually lose money. But in this case, steem is either an over-complicated tipping platform (assuming investors are generous like that), or a glorified advertising platform (assuming investors are not just tipping out of generosity).
Steem has a business model. Just like Bitcoin.
What business model? Bitcoin does not produce anything. It is free, so it does not have earnings. It is a currency, or a open source bookkeeping platform. Steem can be a currency, too, but I suspect the high built-in inflation rate will hobble it as such.
That said, I think Steem works just fine for content curation. The model is quite ingenious, and could eat reddit's and hopefully even facebook's lunch. Just don't expect them to convert into hard cash forever.
Bitcoin is not free. Don't forget the fee.
That's the economic innovation that crypto-currencies have introduced: you don't need to have a profitable (in accounting terms) business model to be able to survive. You stay sustainable by expanding contentiously.
This very feature makes people think it's a Ponzi, but it's not. The key difference preventing crypto-currencies from being a Ponzi is the fact that the outcome of their existence is perceived as valuable - you can't say this regarding a regular Ponzi.
The point is, Bitcoin (or Ethereum or any other major crypto-platform) faces exactly the same problem as Steem: they need to contentiously expand to remain sustainable. Having transaction fees solves nothing and it just a hindrance for the user.
@orly has it nailed, I think. Steem can only have value if people buy it. It cannot have value from being handed out for posting, curating, mining, etc. In the long run (other than speculation), the only reason people would buy Steem is so they can distribute them amongst content creators. This amounts to paid speech, aka advertising.