suicide already is legal -> if you succeed there's no one to prosecute so the question is just absurd. One can only discuss legality of attempted suicide.
You're assuming someone in shape to do it correctly. Someone who is choosing it for medical reasons might not have the strength and motor control, especially if they don't have access to suitable tools.
The healthcare system should not be allowed to suggest it. And it should require an independent review if you request it. That doesn't mean the system shouldn't be allowed to provide it.
The healthcare system is financially incentivized to encourage you to do it. It must absolutely remain taboo for medical professionals to even think about the option.