The Skywalkers are an exception. They fulfill prophecy and are "destined" to play their role.
The prophecy was a PT invention, and was specifically about Anakin. When the OT came out, as far as the audience knew, there was no prophecy ( other than the "Son of Suns" thing which wasn't used in the films ).
Everyone else in the galaxy isn't going to train to be a Jedi, but I imagine any Jedi does have the potential to defeat a Sith.
Everyone else in the galaxy isn't going to train to be a Jedi... Why not? If anyone has the potential to be a Jedi, and any Jedi has the potential to defeat a Sith, why just train Luke? What makes Luke the "last hope" according to TESB, long before the prequel trilogy existed?