There is a question about if steemit is sustainable. But as long as there are people willing to buy your steem off your hands and give you fiat, than it's fine. There are constantly new people coming onto the platform and some buy steem to power up. Those are the ones that are allowing you to take out a profit. Since it's an inflationary coin, when the whole world has become part of the platform, the value might start falling. The trick is, in order for you to be able to give and earn from your steem, you need to have it locked out of circulation as steem power.