There are several ways to curb this problem and it was foreseen by the developers. Have you ever wondered why we have two cryptos on the steem blockchain? Okay. Pay a little attention here as Its quite difficult to explain it in my own words. The answer to this question is in the white papers.

Sbd is a debt token whiles steem is an asset token. Sbd is meant to be dragged to the mud and stick to the 1$ cap whiles steem is meant to move to infinity. That's the reason why you receive sbd as rewards. When you convert your sbd to steem, you actually push the price of steem a little upward. So exchanging sbd for btc does not have a bad influence on the steem ecosystem. The deciding factor is when steem gets dumped on exchanges. This does an irreparable damage to the market. All that been said, the faith of steem is closely link to btc as one cannot convert steem to fiat and vice versa directly. A direct exchange will bridge the slight dependence of steem on btc.
When steem becomes more mainstream, btc would rather depend on steem as it will be easy for someone to acquire steem and use it on real time than btc which has no community support. I hope I have cleared out some doubts. Please read either the steem blue paper or white paper to get the full understanding to the ecosystem