Medical transitioning can alleviate severe distress in some people, and if they are adults and willing to pay, why should we stop them?
On the admittedly tiny off-chance that you are asking this question in all seriousness, I'll give you a serious answer: we should stop them because an individual's health is all of society's concern. And that goes for whether we have a socialised healthcare system or an insurance-funded one.
For example 1: If Individual X refuses to get their child vaccinated for measles, that does not just impact the individual child and individual family. It affects the whole of society because if sufficient numbers make this so-called 'individual' decision, then society would lose its herd immunity to a disease which can maim, deafen or kill.
Example 2: If Individual Y has an eating disorder so severe that their mobility is eventually compromised by their obesity, that is not just a problem for that individual. There's a deleterious knock-on effect to their family and friends, whilst more public money needs spent to care for that individual than if they had been helped earlier on in their lives with psychotherapy combined with weight-loss intervention.
Example 3: If Individual Z has a mental illness so severe that she believes she needs to have a large portion of skin and muscle from her arm removed and fastened into a flesh tube that will be stitched to her mons pubis, and that surgery is so physiologically unrealistic that it requires 3 corrective surgical revisions (the average number of revisions required in the US per WPATH), results in two serious infections (the average number of infections post such surgeries in the US per WPATH), has a total recovery time of 12 months (including the extensive physio needed to regain use of the hand at the end of the mutiliated arm), and doesn't cure the individual's mental illness (as per the Swedish study), then this has consequences for society, not just the individual. Firstly, valuable doctor, nurse, operating theatre, medical equipment, therapist skill and time is taken up for a set of medical treatments that were medically unnecessary and damaged the patient. Secondly, the knock-on effect on the mental and emotional well-being of the patient, friends and family, all of whom may themselves need counselling (watching a healthy loved one deliberately injure themselves can be very traumatic). Thirdly, the patient's lengthy economic inactivity and the possibility that they may not be able to become fully economically active due to possible permanent disability. The knock-on effects to economy and sociaty of that. Fourthly, in a socialised system it ties up resources that others may genuinely need. Fifthly, in an insurance-funded system it pushes up premiums for those trying to fund cover for genuine health needs.
When all that was needed in the first place was decent psychiatric and psychotherapeutic treatment of the underlying mental illness which led poor Individual Z to think that an unnecessary experimental surgery would cure her mental illness.