It seems like a funnel that transfers wealth from people that buy into it, into the accounts of the most popular posters.

It's more like this: Steem constantly transfers wealth from those in the Steem ecosystem who are passive and do not contribute to those who are active and contribute. You contribute by posting your own content or by curating other people's content or both.

But you are right in some sense: there needs to be about 10% annual growth of the Steem ecosystem for it to be able to sustain the equilibrium. If the growth rate exceeds the threshold (around 10%) even those who are passive will benefit (but those who are active will benefit even more). However if you are passive and there is no growth, your investment will have a negative rate of return.

The good news is that achieving a 10% growth (or higher) is very plausible for a start-up, especially in the crypto-space. After a couple of years, when Steem can no longer be treated as a start-up, the economic model will have to be adjusted so that it does not rely on its own expansion so much - but at this stage the economic value of the underlying monetary system (Steem dollars) will be huge and probably easy to monetize (imagine millions of people having millions of Steem dollars to spend).

For more details please refer to the white-paper.