finasteride works forever. I mean 20 years from now its going to still lower your DHT just like it did today.
IMO the reason finasteride stops working for some people is exactly what hamilton found many decades ago. As you get older the sensitivity constantly increases so less amounts of DHT can be enough to cause hair loss. Eventually in some people the DHT is still enough to cause miniaturization.
We desperately need a better understanding of what causes sensitivity and what controls it.