The clinical management of gender dysphoria with hormones and surgery can be regarded as a palliative treatment. Sane clinicians who provide this treatment do not believe that it cures gender identity disorder — any more than an oncologist believes that morphine cures cancer — and patients who undergo it do not necessarily believe that they are going to change sex. Palliative treatments are common in medicine, although not usually in psychiatry (although a case could be made that some talk therapies are basically palliative treatments).
It is a non sequitur to assert that sex reassignment should not be used to alleviate gender dysphoria because there are no analogous treatments for other types of mental disorder. Treatment modalities are evaluated by their effectiveness for a given condition, not by their resemblance to other treatments for other conditions.
Sex reassignment has become controversial, probably because of the vastly increased numbers of biologically male individuals who are now presenting as female. The major flashpoint is the entry of male-to-female transsexuals into “women’s spaces,” a prime example being women’s washrooms. This is a moral, political, and cultural issue, not a scientific issue, and the acceptability/unacceptability of biological males in women’s spaces must be decided on the former grounds.
For some reason, many gender-critical people and many trans activists do not base their views regarding sex reassignment on moral, political, or cultural arguments. They tend, instead, to assert hypotheses (presented as facts) about the etiology and treatment options for gender identity disorder and to base their conclusions on these assertions. It is possible that debate in this area has been so unproductive because people are arguing about the wrong things.