There has not been enough exploration of time-limited online forums. I think knowing a forum would exist only for 1-3-5 years and then be erased or frozen would give it some strengths. I have known only one forum to have done that and I think it helped them stay focused and productive.
The existing model is: a forum acts like it will never end while experience shows will always fade out, have a technical catastrophe or be silenced.
Maybe experimenting with time-limited forums is worth doing.