I can confirm, for me, that atrophy is a real thing. Prostate removal 12 years ago for cancer. For medical reasons that I approved prior to surgery, they had to severe the nerves to get my prostate out (a lot of detail to write here), which left me completely unable to get any sort of erection at all. 6 years prior I lost one of my testes from a bad reaction to a vasectomy and then lost the second one 6 years ago. Then length has shortened some, but the only thing left is the urethra and the head. There is not muscle or other tissue there. It almost just disappears when I put my underwear on and especially so when I lay on my back.
Nothing worked, so we stopped trying to get it to work.
The one positive it that it is quite comfortable not having anything between my legs, especially when it is hot outside and I am out golfing.