You're making a mistake. Apps have a fixed life-cycle. Once everyone who wants your app has your app (happens quicker with niche products), then it will start dropping in ranks.
You do have a point there - but this is an app which appeals to almost all medical professionals.
If you look at the top 10 apps in the medical section (or the top 25 for that matter), you'll see that they have been there for months (or years) on end.
My feeling is that the medical section of the app store is fairly different from the games section (where apps have a very limited shelf life).
Two of the apps in the top 10 paid medical are mine and a further one in the top 20 is mine. I know what I'm talking about. Free apps have a much accelerated life-cycle.
