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.
And what if I tell you that Tor was made by CIA...
Now it is developed by geeks and nodes, relays and bridges are sustained by privacy geeks. They need it for some of their operations. I bet that this is the very same reason why Gavin was invited to CIA - they were looking for anonymous global payments system for their operations so they invited main developer to consult it. Maybe they are using Bitcoin for a while