In my mind, it is inevitable that Silk Road will either be shut down or cease to operate in the not too distant future.
While successors will eventually spring up in its place, there will still be a large, sudden drop in demand for BTC.
How low will BTC go?
How soon might this happen?
I don't believe Silk Road is going to crash. The people who run it seem to be good techs who are working very hard to make it a success and keep it running while protecting their users. More likely, some government will come after them and shut them down.
Meh
I don't believe SR is large enough to crash Bitcoin. I don't think we'll even see that big of a dip in prices. The biggest threat I see to Bitcoin prices are continued scams, the lack of security standards at exchanges, and public gullibility. That's going to contribute to massive issues with Bitcoin prices long before SR going away would.