AGP fails Occam's razor for one thing. Blanchard's theory says there is a causal link, but the causal link goes the other way.
This statement is in two parts. Apropos Occam’s Razor, this claim is unsubstantiated by the OP as no simpler, more obvious alternative is suggested.
For the second part, ‘gender dysphoria’ is not a single thing, it contains elements of depression, obsessive behaviour, attachment and other disorders.
Blanchard suggested transsexuals generally fall into two types, homosexual transsexuals who are less likely to have an autogynephilic history, and those who are non-homosexual (including bisexual and ‘asexual’ individuals) who have a history of autogynephilia. AGP most commonly arises around puberty and competes with the AGP’s heterosexual orientation through their life. The AGP usually transitions late on in life having married, fathered children etc. It is not uncommon for the AGP to transition following a significant crisis.
I’d ditch AGP in a heartbeat if it were a worthless idea, it does however appear to explain elements of trans behaviours very well and predictions the theory make seem to be bear out in the real world: you only need to take note of the words and interests of trans individuals on social media to see real life examples of this type of behaviour.
I find it terribly sad that transgender activists lie to each other about AGP. If they were honest with each other, they’d attain a level of personal understanding that’s presently denied them.