It stemmed from anecdotal evidence of guys on the various TRT forums in which I am a member reporting reversal or improvement in their hairlines when they got their E2 levels under control (generally boosting DHT in the process through feedback). I experienced this myself. I can point to no fewer than 10 other cases where this was directly indicated.
The theory is also supported by comparing teenage men versus old men. A young man in his late teens/early 20's is absolutely chock full of DHT, much higher than the average adult male, yet very few of them exhibit signs of hair loss or prostate issues. Why is this? On the contrary, these ailments do not present themselves in most men until they are approaching middle age, carry a nice spare tire, and with T & E2 levels rivaling their post-menopausal wives.
Nobody looks at your typical middle aged man with his gut hanging over his belt and balding up top and thinks "MAN, THAT GUY LOOKS LIKE HE IS JUST FULL OF TESTOSTERONE!".
I am fully willing to admit this may not hold up to peer review in the end, and don't claim it to be absolute truth, but there is plenty of evidence to at least convince me that DHT is not the bad guy here.
Why do you feel that DHT is to blame for these ailments?