Didn't actually consider that as much for people that go completely offline, but for people that could just have a hiccup that puts them down for a couple of days shouldn't matter, but i think most people don't unvote nearly as much as upvoting. I more so think that specific list should focus more so to see who is currently active and than using other sources such ashttps://steemian.info/witnesses to see the more detailed statistics to see who should maybe not have your votes anymore.