Uh...every time there is a new post/response in a thread, the whole thing will get 'bumped' to the top -- it's not "brand new thread" but "brand new post" that does it. Total post count doesn't matter. (Did that make sense?

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Not that it's my business but I guess I'm somewhat confused as to why you might want your old/inactive thread(s) to stay at the top of the list?
If it's just for your own reference, an alternative would be to bookmark the specific url. But if you want more/new perspectives, just post an update in your (old) thread and it'll appear at the top again.