You know how Mark Hamill knew this? BECAUSE HE'S AN ACTUAL JEDI.
This was all covered in Empire Strikes Back when Yoda was training Luke Skywalker. D'uh.
So Jedi Knight / actor guy Mark Hamill was doing an interview all the way back in 1983 and discussing the future of Star Wars.
He was asked whether or not he'd want to play the role again in a few years time, describing an Excalibur-passing the baton situation.
Take a look.
So there you have it. George Lucas and Mark Hamill are actual Jedi who can see the future.