Hero Member, Newbies and Legendary members do not lose their rank if activity dips below the threshold. This has been talked about before and if you decide to investigate, a simple search in Meta will allow you to find the threads of notability.
I've been on this forum for about two years and have never heard of an activity dip. And this reply made me believe so as well.
Ah, so activity cannot decrease. Good to know.
Anyway I read that it can drop if posts are trashed or deleted ? Just trying to understand how things work.