However, since authors keep selling Steem for fiat currencies, sooner or later all the Steem will leave the system
I highly doubt that. Nothing disappears into thin air when it is sold. It just gets a new owner.
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Yes, the new owners will be the exchange accounts - Poloniex, Bittrex, etc. Would you buy Steem from them to upvote user comments? No, right? That's why Steem will be forever printed and will lose value, unless value is invested back (through advertising, for example).
In the end, the winners will be the domain name and content holders and the exchanges, holding all the Steem. Unless Steem losses value and then the exchanges will be the losers.