I notice that there are relatively few who remain with a project for more than a short time. As I look over the list of names in the group of which I am a part, I see that the great majority have zeroes in all columns and are clearly no longer contributing. I assume this is true generally. I think it might be an incentive for people to stick with it if you could have a field called time-on-project or some similar title that would show the accumulated time a party had been actively contributing.
Obviously, people go on vacation or there may be equipment failure, or the system may fail to update, so up to a month absence (daily update of zero) could be allowed before the time-on-project timer would be reset.