Does Finasteride help strengthen / prevent all regions of head hair from falling out?

Medications like minoxidil and finasteride affect hairs that have altered hair cycles. This is typically the areas that are thinning. Minoxidil and finasteride don't typically affect the areas that are not thinning. These medications don't usually lose potency per se, but the patient may express more potent genes that cause the androgenetic alopecia to progress faster. 

Finasteride can be effective for all regions of the scalp, greatly slowing down hair loss and in many cases resulting in follicles growing new hairs. This includes the strengthening of hairs which are going through the process of miniaturization. The exact same thing can be said for Minoxidil. Both treatments tend to be more effective in the midscalp and crown areas, which probably accounts for why the studies focused on those specific areas. 
Unfortunately, patient feedback leads me to believe that Finasteride and Minoxidil can become less effective over time, so that could be why your hair has started to thin after 12 years of treatment. Your hair does look very thick, especially for a man who has been losing hair from an early age. Consider yourself very fortunate!