I think the hope of censoring bad views is to increase that friction before people get there; make it harder for 'ordinary' people to stumble across vile sites, and reduce access to vile sites to those determined to get it.
I'm concerned that having a hammer of "censorship makes this problem goes away" makes it easier to see other social problems as nails. One line to draw is "censor speech which directly incites or leads to violence". It seems the line NZ is drawing (and ISPs preemptively drew) is "objectionable" or "injurious to the public good". That seems much easier to abuse.
I know some people who will feel that blocking these sites was an affront to their identity. And like it or not, the number of people who feel this way is not small, and their influence even less small. You can't just stamp them out, and you're radicalising them further.
In fact, though, they won't be cut off. 4chan and 8chan are both accessible to me here in NZ with no tricks.
In Australia I believe it was blocking at the IP address level, so requires a VPN to circumvent. But only for some ISPs.
So ... it does work. People don't radicalize on their own, they do so in a community, and they behave according to standards in the community. Sites like 4chan and 8chan are especially deceptive since it's a mix of people (not just white supremacists) who will slowly radicalize (part of) the general population there.
This makes it different from StormFront and all the explicit neo-Nazi websites. You don't go to StormFront to post memes fur teh lulz, only to get slowly drawn in to a white supremacist world.
As for your "sliding slope" argument, literally no one is confused about the difference between literal neo-Nazis who rant about (((replacement))) and everyone else. The entire point just isn't relevant. It's a red herring. It's like "they'll ban cars next!" when discussing speed limits. No one is confused about the difference between speed limits and banning cars, either. It's just not what's being discussed.
This isn't some small event, everyone already knows about it.
There are many possible solutions, but each of them invites a further question: "is it worth it?"