Why did ForestHillOnline forum fail?
Because there was no demand for a second forum in Forest Hill and the administrators, having set it up purely to compete with SE23.com lacked the motivation to keep it free of spam and out of the 6 frequent posters I suspect that at least 2 were the same person, and I wasn't there out of choice. I've also noticed today that Perry Vale forum is currently closed.
Some days on SE23.com are quieter than other, particularly one of the busiest online shopping days before Christmas. But if you think it is genteel then you must have missed the discussion over the pools, jerk chicken, Jehovah's Witnesses, religion and evolution, and most posts by Nasaroc, Roz, Hilltopgeneral, and Brian. You probably also missed the legendary discussions between ToffeeJim and BaggyDave back in the days when SE23.com was young, free, abusive, and making enemies out of every shop in the area.
SE23.com is actually a lot more successful than you might give it credit for and regularly makes a big difference in public campaigns such as the Olympic badges for Horniman Museum (STF also helped of course).
As for long threads, I would prefer to have a single thread on a topic that goes on for a long time, than multiple threads where roughly the same discussion is taking place, often resulting in a series of monologues rather than a discussion. But everybody has their preferred way to use forums.
I'm not going to defend every way in which SE23.com is run, but just like STF it is run by somebody who spends quite a bit of effort keeping it going in his own way. If you think you have what it takes to set up and run a better forum, then why not do it?
But if you really think STF and EDF are so much better, how come you had not posted to either before you started these threads? It does suggest that, whatever criticism you may have of SE23.com, you have not spent much time on these other forums.