Well, I'm just now seeing the edit, so sorry for the really late reply. However, heritage.org is extremely biased to the right, whereas I sourced peer-reviewed academic research. They just have a doctor with a personal agenda that basically just says "trans people aren't really changing sex", which trans people don't disagree with, yes trans women are women and trans men are men, but biologically we still understand there's a difference, we're not delusional, but that's just not really the important part. What's important is that people feel 'normal' with transition and acceptance, if it brings mental health in line with the general public without doing any harm, I don't really see a downside.

The studies they're referring to are described as such: "The quality and strength of evidence were low due to the mostly observational study designs with no comparison groups, potential confounding, and small sample sizes."

Small sample sizes and potential compounding. This is important, because if they have a compounding variable in their study then the data is meaningless.

Also, the study looks specifically at gender reassignment surgery, which can have a lot of reasons for why someone might regret it. Post-surgery depression is very real and many people who undergo GRS experience it. Their results might be less than ideal (which if we're being honest, then in 2004, when the cited Guardian article was published, the results probably weren't stellar.), and the list goes on.

Studies have also shown effectiveness, there are studies which include GRS in my previous comment.