I don't think if that is possible to delete almost all your post, but that not withstanding your activity and post will increased whenever you completes the 14 posts per say or it will start counting afresh which might take longer time to get to ur previous position. What determines your ranking up is your activity and you can only get activity when you consistently keep posting.
since the activities are responsible for ranking up and I know it can't be deleted, is it possible for that to also happen?
Who told you it's not possible to remove activities? When you delete a post that has been updated with activity, those activities will be reduced to your current post count. Even if your activities are what qualifies you for your current rank, they can still be reduced, and if you are short of activity, your rank will automatically reduce by itself. Let's say, for instance, that you need 120 activities and 100 merit to be on the full member rank, and you have already achieved those 120 activities, so you are already seeing your profile updated to the full member rank. Then, if a few of your posts are deleted by the moderator and you happen to run short on activities, it will just take a few minutes before the system will automatically bring you down from full member rank to member rank until you make a few newer posts that will equal your activities to full member rank.