The thing is even if you don't think GD is a mental health issue, it tends to co-exist with MH issues (I know there are debates about why that is) and for that reason alone, affirmation is just a terrible idea in many cases.
The trans person i know - female, identifies as non-binary but this seems to involve trying to look male - has spent their whole life with MH problems and leaping onto any bandwagon and getting any diagnosis they can as, it appears to me, a way of pursuing a sense of self. Them announcing themselves as non-binary didn't make me think they had GD or were trans in some mystical, inner-soul way. It made me think "another bandwagon" and I found it extremely predictable. I'm not trying to be mean, and I'm sure there are better psychological terms for this phenomenon, but they will do anything to have a new tribe to be part of and a special status.
It seems blindingly obvious to me that the huge rise in people identifying as trans involves a large number of similar situations – especially given the atmosphere in which everyone is expected to bow down to trans demands and no one is allowed to question the logic of it. Of course that's going to appeal to insecure people with low-self-esteem, confused teens and bandwagon-hoppers.
It's nothing like being gay because being gay does not involve a faith-based claim about being something you are physically not, and demanding that other people agree. It's also very different because announcing you are gay is fully reversible and doesn't involve a risk of harming your body. It's just a preference/lifestyle.
And those who want to resist the affirmation model are nothing like those who thought / think conversion therapy for gayness is a good idea. Why would a bunch of lefty feminists and lesbians, the type of people who marched against clause 28 and fought for gay rights, be that kind of person? Answer - they're not.