There are plenty of people who thought that it wouldn't work to that extent, either. They claimed that allowing a subreddit for toxic topic X at least had the benefit of segregating proponents of X; and that if such subreddits were banned their inhabitants would infect the rest of Reddit. But that didn't happen.
I remember when posts from r/FatPeopleHate would regularly make the front page. Then FPH was banned, and you know what, we didn't see it metastasize elsewhere, we just saw many fewer FPH-style posts, which I'd say is a good thing.