At the moment, it seems that SXSW will still happen. According to a statement on the festival's website, officials are "working closely on a daily basis with local, state, and federal agencies to plan for a safe event." Even if the festival does start as planned on March 13th, it will be considerably smaller than it has been in past years, as large and small companies alike attempt to avoid putting people in unnecessary contact.
Several other festivals and tech shows have already been canceled, including GDC, MWC, I/O, F8. Today, organized postponed the upcoming Minecraft Festival, and chances are we'll see more cancelations. As Engadget's Jessica Conditt points out, panic around the coronavirus outbreak could expedite the demise of in-person tech conventions altogether.