It's the same thing with every inflationary currency. For example, if the market cap of Bitcoin stays the same, it means that price of bitcoin will go down because new coins are mined continuously.
New owners (miners) benefit while the value of coins owned by old holders will go down.
Of course the market cap will go down if people sell their cryptocurrency for fiat. But the same happens with Bitcoin, too, when miners have to sell their coins to pay their electricity bills and buy new hardware.
Compared to Bitcoin, Steem has advantage if the market place will come someday. It will create larger internal economy where people don't have so big need to exchange their SD to fiat currencies. Instead they can use them inside the Steem ecosystem.