I don't think that the eternal youth surgery will work without a sacrifice or it was insignifcant when it was brought up in climax of doffy's fight
An eye for an eye
But its usage being on a protagonist (luffy or any useful person for the D) as a last resort against the big bads or being used on an antagonist (probably blackbeard) as means to defy a deadly force both fit the narrative as of now
An eye for an eye
But its usage being on a protagonist (luffy or any useful person for the D) as a last resort against the big bads or being used on an antagonist (probably blackbeard) as means to defy a deadly force both fit the narrative as of now