[the_watcher]

I assumed some knowledge of Hamill's personality (or at least the one he cultivates publicly), which is that he's a pretty big ham for this sort of thing - long attendee of fan conventions, well-known for indulging in fan service, etc. Essentially, I might agree with you for an average person, but for Hamill specifically, there's a decent amount of evidence that he'd appreciate this.

[bredren]

Apart from personality, fan engagement also drives fan enthusiasm for HIM as the on-screen portrayal of older luke. That's essentially leverage and increases his influence.

[ozaark]

The key was someone who knew him well enough to suggest a prank to begin with. That was where it became possible to delight a user with such a great outcome.