For those of us who have had this problem (myself included), I think I may have a - partial, at least - explanation for the issue.
In my theory, the “brain-dick connection” is the nerves and the sensations which provide you not only with sensation, but with pleasure of that sensation. This can be a dopamine response to penile sensation. Dopamine is working to not only tell you “um somethings on your dick”, it’s also saying"…and it feels fucking great". Without dopaminergic transmission between neurons, your penis will not feel “connected” to your head - aka your desire for sexual intercourse.
Also, as I have mentioned before, ball size is key. I assume this implies androgen, specifically T, is needed for not only sex drive, but to implore your penis to become engorged with blood and, thusly, thicker. In a state of hypoxia or blood loss, cells presumably do not thrive, they die. IMO nerve cells die off in this state as well. In a hypoxic dick - one that does not get filled with blood, what’s left of the penis? Just the thinning corpus cavernosum (which I believe grows anabolically with the more T you have) which will be “retracted” (need to investigate this terms relevance in male Urology, since the Uro’s assistant said mine looked a bit retracted in 12/08) and the lot of skin that will wrinkle. Your balls may also sag due to not being active.
Anyway, this is my explanation. There’s more to everything, but I think this aspect is crucial.
I personally am getting larger erections - not hugely throbbing ones but good enough for sex - and feeling more on the penis as well, the more I get an erection. So I also believe this is not a permanent state and can be regenerated and restored in time.
(BTW I didn’t address shrinkage all that much, but I believe it is real as well - I have had it, and continue to recover from it. Hypoxic dick probably leaves less thriving environment for androgen to make proper amount of thickness, so it shrinks because of that too).