You can't build a selection rule based on inactive stake. The canon chain may have a whole load of inactive stake during genuine quiet periods, you don't want to select against it during these times, because that's another attack angle.
20% active stakes suddenly become quiet at the same time and never active again, that's unusual. Why can't it trigger any alert when we are analysising?
That was an example, what I want to say is : you can't easily fake a perfect chain that can't be recognized when we compare it with the real one.