The Star Wars main theme actually was developed as Luke’s theme and is used as such in the original trilogy.
It started out as Luke’s theme in A New Hope, but quickly evolved into its own theme throughout the movies. So I don’t have any sort of problem with it being used for moments where Luke isn’t on-screen. I mean, the VERY FIRST TIME it was used, Luke hadn’t even been introduced yet, so it’s pretty much a moot point in my opinion.