No, I don't think anyone should be de-ranked. What you have contributed remains what it is, it should not be taken away.
Wrong. Activity/ranks =/= contributions. Then again, given your post history I am not surprised by this comment.
De-ranking is actually a good idea; it goes well with merit points (i.e. more damage to the rot of this forum). However, the proposed requirement is bad.
Or you could simply implement a minimum merit/month feature to maintain your current rank, if you don't earn a certain amount of merit within the month you drop to the previous user level......
Seriously dude, do you think satoshi's account should get demoted because he doesn't use it anymore?