I think one factor which kills off a forum is when it gets cliquey i.e. when the top posters know each other personally and use it like a pub. The flyer forum is a lot like a pub now, but the clearest example is ukga.com where practically every thread degenerates into drivel very fast - they all know each other (in fact I gather that forum was started to support a particular airfield community; Sherburn or perhaps Cardiff?).
What I like about pprune is the lack of clique. There isn't much need to read between the lines. I do this stuff for fun while having a cup of tea, and don't have time for posting thousands of one-liners. OTOH I know many will disagree with this, enjoying meetings, fly-ins, etc.
The Guardian article is unsuprising. I know of one forum where people do mostly use their own names, yet I have seen some vicious attacks there. Presumably because they think they will never face the target. However, I would never have anything much to do with the real people afterwards. Also, with nicknames, you never quite know whether the target of your attack might be somebody rather closer to home
Thankfully there are much fewer attacks here now, and it's fairly easy to deal with them.