"Right now, Steem has value because you can exchange it for bitcoin": this isn't strictly true, although it's a position likely to be taken by a bitcoin maximalist. All the arguments used to explain how bitcoin can provide value without fiat can equally be used to argue for Steem (or indeed many altcoins). Bitcoin has two major advantages right now: 1) it's the main gateway for exchange to fiat and 2) more merchants will accept it than other crypto. But of these two advantages, I believe it still mostly comes down to 1) right now.