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As mentioned, the Activity will decrease and, subsequently, you may be de-ranked depending on how many posts you delete. The good news is that the Merits will stay, so in the hypothetical event depicted in the OP, it would only be a matter of posting (and time) to recover the Rank. Bear in mind that Recoverable Activity would not be available as an accelerator to recovering Rank if you delete all your posts.
In broader terms, some people have managed to de-rank their account from Legendary to Sr. Member and even back to Newbie, as well as others have taken their Hero member account back down to Sr. Member through to Newbie.
Not that I frown at people deciding to delete their posts for the sole purpose of de-ranking themselves and on the other hand it doesn't in anyway fault forum rule in doing that.
But am just raising a concern I conceive in mind after reading your comment, like in a situation where an OP gets to delete a particular post of his which contains quality information, solves certain regularly asked questions that might be of a burden to a newbie or any member rank at all. So to me I think if many people continue doing that we might lose some answers we could have easily sight or pointed to in the forum when we meet similar questions that such reply/answers could have resolved or been of help to but then it already deleted by the OP.
I don't know if you understand the perspective am talking from.