Cloudflare did drop 8chan for political pressure.
Both also have a very undeserved reputation. I saw news reports on 8chan as if it were something similar to Stormfront. The front page was mostly video game and pornography and one had to search very deep into it's hidden circles to find racial supremacy content.
ISPs could just drop their customers' traffic to that server. This happens all the time and has happened to 4chan before.
Or they could go further and refuse to peer with any ISP that doesn't block 4chan, or lobby to have the government compel ISPs to block it, likely on think-of-the-children or think-of-the-terrorism grounds.
Self hosting definitely isn't some magic way of preventing all censorship.
There is no perfect pill. People who self host have higher protections than those that don't.
With TOR you have to hire someone to fill captchas just to read things.