But in the exceedingly unlikely event that SR was to shut down or be shut down due to some previously unknown weakness in TOR itself, that cannot easily be patched, the long term price impact on bitcoin will indeed be quite negative, since untraceable trade is one of the major bitcoin value propositions.
Since TOR is used by our own government so it can get away with whatever fucked up shit they do anonymously over the internet, I don't see that happening. Sites on TOR even survive DDoS'ing.