There's a strong chance that the racists and sexists will eventually kill the site. They won't do it over night (like many of them are used to), but over the long run, the site will die.

If Reddit takes anything but a negative stance on racism, it just kind of tells the racists to come on in. There's already a lot of defaults that /r/coontown and the rest of the chimpire are comfortable with, but if more keep pouring in, then Reddit's going to get pretty saturated (or unsaturated, since it's a white supremacists movement) with racism.

The racists will love it, but a lot of the regular users will not. Like a few that are here now, many users will move somewhere a little less hostile - which will leave the racists unchecked. They'll start gaining traction, which will make more people leave, and the cycle will continue. A couple of news sites will start to catch on to the changes that will happen and Reddit will get a lot of bad press (as in, more than usual). That bad press, combined with new users being turned off by the racial tension on the site and the lack of people recommending Reddit to others, will slow and eventually stagnate growth over time. The site will no longer be able to counter the natural decay of a user base after it's peak, and the site will just wither away.