You're only wrong here in that you think McCain was the one who made this decision. If he didn't have a multi-generational military pedigree he would have been a community college graduate in Arizona, and nobody ever would have heard of him. He was probably one of the dumbest Republicans in Washington for more than three decades, and he surely wouldn't have known enough about the field of prospective VP candidates to pick one over another.
Every politican has advisors. Trump had to choose from what his advisers recommended, since he had no clue about people in D C. Just like you would have to if you were up there.