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.
I think yes, you doing it would be unethical as you cannot get the consent of all the people you’d be affecting. Making the option available would certainly ease suffering but your blanket statement “that’s what they want” assumes something that certainly isn’t true for every single trans person.