Think of Steem as a pump that does not create any new value but constantly re-distributes the existing value (or shares) according to the perceived value of input created by the users (or shareholders).

Your first impression might be that such economic model cannot be sustainable, but deeper analysis indicates that it is sustainable as long as Steem ecosystem expands at a rate faster than 10% a year. The case of Bitcoin (or Ethereum) shows that 10% growth rate is realistic, even in the long run.