That's why I'd like DiffuseThinner1 to elaborate on his theory, I'm not sure why he thinks that. Be great if true, but....love to know more
This is fairly late in the process, but if I may, perhaps I can answer this to the extent of what I know. Anecdotally, i took finasteride particularly because i was going through diffuse thinning systematically across the top of the scalp. I am pretty glad with the decision I made, and overall I have noticed over the near two year period of time a progressive thickening and improvement in overall hair caliber (caliber being that the hair "sits" better on the scalp and feels heavier than the straw like texture that it was accustomed to being)
when it comes to diffuse thinning, it is really a double edged sword in a multitude of ways. First, you are in a relatively better aesthetic position vis a vis a traditional individual moving through male pattern baldness, as the hairline remains in tact (I still have the same hairline I've had since I was a teen). The issue here is that it also is a silent progression because you can't really know the extent until it actually hits you. When you actually can tell that you lost hair, you likely lost ~30-40% overall density within the affected area. This makes it difficult to really catch it early unless you know for certain that there is strong likelihood that hairloss runs in your family and you make the necessary due diligence with your doctor to track your scalp for miniaturization. On the plus side, the good news at least in your unique scenario is that diffused hairs are usually many many vellus and thin hairs that are virtually transparent when taking photos unless you take very hard looks at your scalp. because of this, you may have above average opportunities to recapture some of your previous density and reverse the process to some degree.
To the degree of realistic expectations (in the case that you aren't already on finasteride anyway), I would say that if you were an average responder you would maintain your current hair with better hair caliber across (hair counts aren't necessarily as important as hair weight, as thicker hair tends to mask hair loss so long as there is not substantially material loss systematically across the scalp). If you are below average, expect maintenance to below baseline for a few years, but ultimately trending to baseline in years 5+. If you are an above average responder, expect arresting of hairloss midway through the first year, regrowth through the second half, and continuing growth over the duration of the antiandrogen therapy (the rossi et al study in 2011 suggested that the effects of finasteride do not subside over time, so if you are a good responder in the first year, you are almost assured to have persisting positive effects with the drug over time. This is namely an increase in hair caliber coupled with an moderate increase in density and hair counts (an average of about 15-20 FU/Cm^2).
You really won't know until you try, but speaking purely from my anectdotal analysis, it stands reasonable that your case of diffuse thinning has the best opportunity to recapture lost density and regrow "lost" (severely miniaturized) hairs from the past 3-4 years. Remember that hair on a bare scalp that doesn't grow cannot be regrown. Hair cannot be created from thin air, but drastically affected hairs can recover over the long run and capture potential terminal level thickness. Overall, I would advise you that its definitely a prudent decision, but a big one nonetheless. As always, consult with your doctor and a reputable hair loss surgeon or doctor before you jump on the drug. I wish you all the best