I suppose the Forum can do okay simply being reliant upon members who choose to engage only when they have something "good" to say and no one is force to do anything more; however, The Forum, by definition, is a space where engagement is encouraged and welcomed, it is also a place where contributions of new ideas and topics are required to maintain a degree of relevancy and newness.
The danger is that ideas die or those who may consider to make a contribution are left without the courtesy of a simply reply or acknowledgement of their effort or even the perception that their contribution may not have even been read..so why bother!
Clearly, the Forum is not guaranteed to survive especially if enough members decide to become idle. Take a look at the Members List section; you'll find 121pp filled with members who have posted 1-2 times and have not bothered to return. Left to upon most of these members, whether the Forum is still active would be news to them.
As for having the need to say something good, that is kind; there is also a need when constructive to point out the not so good.