"reasonable users" is doing a lot of work here. not everyone desires the same level of discourse you do on every site they visit. /b/ has just about the bare minimum level of moderation needed to comply with US law and a lot of people seem to enjoy using it.
/b/ is a good example of a forum that is only trolls at this point, and as you note it does have some moderation.
Some of the other forums on 4chan might be better examples, but even those have some moderation. Forums like /tg/ have managed to hang on alright, although their total post volume per day is quite small compared to /b/.
