everyone in the comment section is just making a good point in their opinion.
Taking in consideration from what @NeuroticFish said
Unfortunately, at a second look, it's no longer that good imho, since this ignores the fact that some smart and worthy people may have a life outside the forum and will punish them for that. Or it ignores the fact that forum users did and will pass away and it will look ugly/strange to see this or that user has became newbie again, somehow ignoring his achievements.
I think the idea would have been a nice one but base on the fact that some people have others things they are doing outside the forum, or a user can seriously be sick and hospitalized for some months, a user might passed away, or decides to further their education, and when all these happens, their achievements in the forum can't just be demoted to a low rank. This is more like a punishment, forgetting the fact that their quality posts was what defined their original rank.