I think this is a good idea, but it could benefit from some specifics. I suggest:
1. In a case where staff/dev team comment on a suggestion, the max. time before a post is deemed 'old' is 3 months. This allows players to continue to provide insights/thoughts etc., but not to a point that is overly-critical or amasses so much information that it becomes a similar case to the UserVoice where they said they found it difficult to consider/respond to everything (not that they are going to respond to everything of course, as that is part of the reason for the switch.)
2. Threads which contain ideas that have been implemented/accepted (and require no more feedback) are locked (perhaps on a case by case basis determined by the Wizencouncil team) to avoid necroposting where it is not necessary.
3. Ideas which are outright rejected, etc, should have their respective threads locked. (This obviously covers duplicate threads as well.)
I hope that makes sense haha, I'd love to hear any feedback on these ideas!