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.
Assuming enthusiastic consent, good faith, and that you meant “sex/body they want” instead of “gender they want” (because gender is just a social construct):
I don’t think it would. Identities are built from life experiences, and having lived through transition they’d still be trans even if there were no traces of it on their body. A war veteran doesn’t stop being a veteran just because the war ended.
The definition of genocide depends on intent! Even in wars, etc. It’s only genocide if you’re specifically trying to erase/displace people/culture.
Trying to cure gender dysphoria: it’s not genocide, it’s medical treatment.
Trying to “fix” people to make them fit into society: it’s genocide.
There are identities that don’t stop being trans even if you give them the body they want:
A non-binary person’s desired sex/body and social gender might not match. Even with the perfect body (if one exists), they might still identify as trans because that body doesn’t match their social gender.
For genderfluid people, there might not be one singular perfect body. Even if their body constantly updated to suit them, they’d probably still identify as trans because they’d be constantly transitioning…