I don't care about legalism and I'm not going to get into gotcha debates about, "well technically you said".
My intent behind that statement would be more accurate to say: non-affirming environments force children and young-adults to make extreme decisions without the benefit of experimentation or counseling over those decisions. In that environment, it is more likely that they will regret the choices they make, because they will never get an opportunity to try out those choices in a limited capacity beforehand. This seems to me to be an obviously logical, reasonable statement: people who have less experience living as a trans person are less equipped to make weighty decisions about their future life. People who have less experience in any area of their life are less equipped to make weighty decisions about that area.
If that seems like a different argument to you than what I said before, then I apologize for being confusing, clumsy, and imprecise, and I'll happily concede one internet-point. But beyond that I'm not really interested in those kinds of fights.
> The answer is that I care whether my kid will come to have regrets. I don't actually care whether they are trans. I care about whether they are happy.
In which case, the only studies that will be helpful are the ones that measure regret, not rate of surgeries. Unfortunately those studies are limited. Bear in mind that post-puberty regret and regret over waiting on transitioning are also possible outcomes here. Both medically transitioning and waiting to transition or avoiding puberty blockers/hormones can have negative outcomes, which is why I emphasize kids being able to get more life experience being trans before they make that choice.
While long-term studies on transitioning are limited, there is at least pretty strong evidence that affirmative care reduces suicide rates in trans youth[0], so it does seem to have some measurable positive outcomes.
[0]: https://www.thetrevorproject.org/2020/01/29/research-brief-g...
"That's not really what I was getting at, what I was getting at is that creating an environment where people refuse to use proper pronouns or provide even basic accommodation for trans kids is going to make them more interested in medically transitioning, not less."
I don't know whether that statement is correct. But my strong suspicion is that it is the opposite of the truth. That the more we encourage the idea of transitioning, the more interest we create in all forms of transitioning.
Moving on, you ask:
"Do you care if your kid has regrets about the decisions they end up making about their body once they reach adulthood? Or do you care whether your kid is trans? Because those two concerns are completely orthogonal to each other."
The answer is that I care whether my kid will come to have regrets. I don't actually care whether they are trans. I care about whether they are happy.
It is my belief that medical interventions in teenage years are likely to work out poorly for future happiness. I wish I could say that this is an informed belief, but after attempting to inform myself I concluded that good information is not actually available. Still, since that is my belief, I believe that increased odds of a medical intervention implies increased odds of a bad outcome.