Theymos was talking about this in the original thread, I don't feel like going through the entire thread trying to find his mentions of this in particular, but from what I remember this was just talk about possibilities in the future.
Someone, please, correct me if I am wrong, but
there is currently nothing in place that decays merit at all. This disclaimer was probably put into place, because theymos' sees this is a very real possibility for a future implementation within the merit system. For example, inactive legendaries coming back and slinging around merit like they're a pinata, I'm sure would be something that would be a contributing factor to this sort of thing.
I think the ranks should also be reduced if the member is not able to get certain number of merit points in a fixed time interval.
This doesn't sound fair, because these members have already solidified their place in the community; Satoshi, if he were not in his own rank, would fall into this category of member that have contributed in the past and deserve to maintain their rank, regardless of their future input.