I've actually studied acting for years. I've seen a lot of terrible actors and people who are good and struggle to get parts.
On observation. One of the biggest things to being a good actor or at least not a terrible actor is to learn that acting is much more about reacting. Learning to listen. Stop reciting lines, and instead LISTEN to the other people on stage and their needs and desires. Then react to those. A good actor could forget all their lines and still get through a scene because they can just react in character to what's going on around them.
James, as you can imagine from this article, was awesome at just listening. He didn't start out as a great actor but this quality definitely helped get him there.
On taking chances. I know some good actors who are baristas right now when not acting. When James needed money as a starting actor he was helping other actors recite lines, cleaning actors dressing rooms, even washing their cars. Which do you think will get you more possibilities for "luck", making coffee for random Joes, or helping Marlon Brando with lines?
But I agree with you that there are a lot of people trying and they'll never be as successful as James. But I also know a lot of people who won't ever as successful as they want because they do so little to find new opportunities. Also, the article wasn't titled, how to be as successful as James Garner, but was meant to point out some things Wiseman has found "lucky" people (not necessary James Garners) doing that "unlucky" people aren't.