There are children as young as 8 who have medically transitioned with puberty blockers here in the US, and in other western countries like in Scandinavia. There are female children growing up right now with painful osteoporosis and fertility issues due to the impact of puberty blockers at a young age. There's some more info about that in this post.
There are detransitioned women (I know some personally) who were put on testosterone at age 14 and who now speak out against it due to the health and psychological problems it gave them. There are minors (under 18) who have had surgeries.
The use of puberty blockers at a young age in males also renders them unable to feel sexual pleasure as adults. Imagine having that ability taken from you, and having a surgery on your genitals, all before you reached adulthood? Before you even know what sex is or how it feels, doctors think that you're capable of making a decision like that. This isn't fake news – Jazz Jennings is a prime example, her experience has been very well documented, and there are many more young people like her.
I assume you didn't know all that, so now you do.
There are also adult females who develop issues such as cardiovascular problems, including polycythemia/erythrocytosis which can cause blood clots and stroke, blood pressure and cholesterol problems, heart disease, chronic pelvic pain from uterine and vaginal atrophy, pelvic floor and urinary problems related to the atrophy, PCOS, and diabetes from being on testosterone even on a shorter term basis. Plus, nearly every woman on testosterone for more than a few years loses her hair, regardless of her age or family genetics, because female bodies react to high levels of testosterone differently than male bodies.
If you don't believe me, look at the Medical Research tag in my pinned post, look through the links in this post and this post, watch this video, and seek out stories from detransitioners who have been medically harmed by this.
The bottom line is – Female bodies are different than male bodies. The insistence that high levels of testosterone are safe and okay for our bodies is an example of medical misogyny.
The studies done on longterm happiness in transition were relatively short term, and these studies didn't include people who stopped transitioning and stopped going to their doctors. We don't have accurate data yet about the number of detransitioners because it hasn't been widely studied in the current day. Which means that we don't have accurate data about the real percentages of people who stay transitioned longterm (10+ years) and who are happy with it vs those who aren't and stop.
Transition regret also carries a lot of stigma. I couldn't even begin to talk about regretting my transition for years after I started feeling that way. Many people who would detransition if they felt like they could instead feel trapped. I've heard women speak about detransitioning after 20, 30 years on testosterone, knowing they regretted it the whole time.
Do I think everyone will detransition? No. And again, I respect the choices of the people around me, because they're important to me. But that doesn't mean I believe that transition is the right option or that it's something that the medical industry should be doing to people when there are other options for us to explore in therapy (from a feminist perspective!) that can help us heal from dysphoria.
So, I'm against medical transition and the misogynistic, homophobic, and GNC-phobic society we live in. And, I believe those societal problems are the real reason that people transition.