Human definitions and laws are only changed when new evidence points out that it should be changed. Right now all evidence points towards transgenderism being a mental illness.
A half-truth at best.
Laws and definitions don’t change only when evidence dictates. Humans are too emotionally driven for that. They change when those placed in positions of authority to make such changes are in some sort of internal consensus. Also, evidence by its very nature is open to interpretation. Individuals within the same authoritative body might have wildly different views on what the evidence actually means. But, they still have to make a decision, therefore simply majority often wins.
I don’t know enough about the science involved to make any sort of declaration about how to define transgenderism in the eyes of psychology. I do have serious doubts that you’ve collected “all the evidence” on this issue. I also know that throughout my entire life no issues has had “all the evidence” be stacked on one side. The fact that there is even a debate should be evidence enough of this.
I don’t even want to get into how hard it is to get prevailing theories in academia changed (a problem most often pointed out by other academics). A depressing amount of evidence in our scientific community goes unused or unacknowledged. That, however, is a whole other conversation…