I'm critical of censorship, but not because I think it's ineffective.
That won't completely get rid of it, but it can make it a lot less accessible, which reduces the amount of radicalization that occurs. I don't think people start out with the intent to get radicalized. They stumble upon a place like /pol/ and see that it's interesting and so they stick around and gradually get convinced. It's much harder to stumble upon a Tor hidden service or a private chatroom than a publicly accessible webpage.
China's great firewall is not terribly hard to get around, but it's very effective regardless. Inconvenience is powerful.
That said, I don't buy the slippery slope argument. My country bans holocaust denial, and though I think that ideally it shouldn't, it hasn't actually slipped down the slope and the situation seems stable. 8chan currently bans certain content that it didn't in the past (it's now much stricter against photographs of children) and it's still incredibly permissive.
If 8chan stopped allowing posts that promote shootings it wouldn't end up becoming like China. I don't think it would lead to other more draconian restrictions.
The harm of censorship usually outweighs the benefit (if any), but not in this case, I think. It's probably ok for 8chan to change its policies a little - and I do mean just a little.
At the same time I definitely think slippery slope is the right analogy. The definition of what is acceptable speech is constantly changing and IMO there is no telling what the effect of banning unacceptable speech is - over a long enough time frame.
My view is things might seem stable at the moment, but who's to say what the relevant timeframe is?