If you take an online community down, the people that made up that community will not all end up in the same place, they won’t bring the entire culture of that community with them, and some of them won’t make the jump.
Maybe you end up with three new chans, but you end up with three smaller, more fractured chans. Organization and relationships will evaporate because people won’t have alternate ways of recognizing or reaching their friends, may not use the same handles, may not end up on the same sites, etc.
Site administrators will get worn out and tired of dealing with it, and may run out of money to keep the site up.
I’m not saying that you should take down 4chan, just saying that if 4chan did disappear, it would leave a big hole that would take a long time to fill again. That, and any new chans would have a hard time claiming to be THE chan. There are already like twenty active “chan” sites or more. Besides 2channel and 4chan, the other sites don’t have much credibility or “pull”.