Very good observation, however as a good statistician you must remember that we are not analyzing a static situation, and a situation at time T may be different than a situation at time T+1.
So after the first post at time T, your total contribution to the thread may have a high probability of being useful at time T.
But after 10 posts at time t+10, your total contribution to the thread may be judged to be MUCH less likely to be useful at time T+10.
Though I will concede that this does mean that people who decide to leave the forum and boycott it after your first post will be immune from your further rantings and will not experience the decaying of your usefulness. I suppose this creates a practical floor on the usefulness of any given person, since people will stop reading the thread at some point.