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.
Some people are at peace with dying, and probably most terminally ill people have lived long full lives. If they already know they’re dying they’re likely to have made their final arrangements and said everything they needed to say and are accepting of death, a completely natural thing.
You personally assuming that you would want to be magically “saved” without any prior knowledge or consent doesn’t mean everyone else feels the same.