I am not asking this to be transphobic or anything but I had this debate with myself at 2 o’clock in the morning and every time I remember it I can’t focus.
On one hand, it is what they want. Let’s assume it causes no harm to them or any unforeseen circumstances.
On another hand, it would erase their identity as trans people. At the extreme you could consider it a genocide, since turning them into what they want would mean there is no more trans people and their unique identity is erased.
There’s a weird divide between self-determined identity and external classifications. Often, a culture forms around the label and the external label stops being relevant because the term has more social/cultural implications than practical implications. Some people internalize the label as that’s how they wish to steer their future interactions, and others ignore the label and move on with their lives.
You can watch all of Star Trek, and some parts of society will label you a Trekkie if they find out, but it’s up to you whether you choose to identify as a Trekkie, or just go about your life not making a big deal about it.