Finasteride, will I keep more hair staying on it?

People say that Finasteride loses its effectiveness. Is this actually true, since DHT levels are similar till 40? I don’t see the logic of why it would lose its effectiveness. 

Studies comparing two groups (1) people who stay on finasteride vs (2) those that do not take it or stop it show that those on finasateride keep hair counts higher for longer. Some further studies by others seem to show that the spread between the two groups continue past 5 years and from my clinical experience I have seen it extend to 20 years. For those that take it and stop it, they shift to the lower curve fairly fast (3+ months) which I have also seen in my practice.