I seriously doubt that the situation in the Tenderloin can be blamed squarely on rent control. There's only a small part that's genuinely scary, and that seems almost institutionally embedded at this point. Rent control or not, you're going to have a hard time convincing the affluent to move in next to the SRO's, liquor stores, porn shops, homeless centers, and methodone clinics. I think geography is to blame more than anything - it was completely destroyed in the 1906 earthquake, it's not downtown, not on a hill, and not near the bay (where the wealthy wanted to be), but it was close enough to all these things that the lower class that worked for and entertained the wealthy could set up. Throw in the flight to suburbia that hit most cities after WW2, and it's pretty understandable why that part of town is like it is.