4chan.org is the 212th most popular website in the U.S. at time of writing. This implies that it is able to attract plenty of different kinds of people and that the trolls aren't driving the rest away. If you go on most boards it's pretty easy to find interesting content and it's not particularly surprising that the site has remained a staple of internet culture for 15 years. I think that one of the main reasons for this is that the design of the site doesn't inherently require users to think beyond the scope of their individual posts. In general, there is no persistent identity and the visibility of posts is not determined by any kind of like/dislike feature. People post things and there is no kind of long term reward or punishment for posting "right" and "wrong" things. This kind of structure tends to create a community which is a lot less prone to group masturbation and smug and self righteous than certain other internet communities where long term identity and other people's opinions of your posts actually sort of matter. There is value in that idea, even if the that community type it's not for everyone.
It's more than just the absence of a like/dislike system, visibility of posts is directly determined by how much discussion they generate - for threads this means its position on the board stays higher for longer, so more people see it, and for replies the number of replies is visible while scrolling through the thread. This links agreement/disagreement with actual discourse (or, at least, communication), unlike e.g. Reddit where agreement and disagreement are done via voting, which doesn't contain any information other than the upvote/downvote itself. In systems like this, disagreeing via replying is actually suboptimal in terms of return on the labor involved with communicating this disagreement, because downvoted posts (posts people don't like) are less visible, meaning replies to them are less visible as well, even if they agree with the majority of the people downvoting their parent post.
On what grounds? They're just as high (well, low really) quality as Reddit but they're impolite and vulgar whereas Reddit gets angry if you're impolite and vulgar.