If people could purchase Steem with fiat, then I think the correlation will decrease even more. Users still need to exchange from another cryptocurrency in order to receive Steem (excluding earnings from curation and creating content). If people could purchase Steem with fiat, then it drops the reliance on any other cryptocurrency as the reserve currency used to facilitate these exchanges.
Another great thing about owning Steem is that it has intrinsic value (ownership of the network and bandwidth allocation).