I'm not sure that "discussing crypto" should count as single-purpose. Otherwise many of the more interesting people who post on hn are almost certainly single-purpose posters (on whatever their pet topic/project/area of expertise happens to be).

In any case, I found the above post compelling. What if critics of crypto pooled resources and waited for a major crisis, and then launched a 51% attack at just the right moment? It's an interesting thing to think about.