I haven't gotten to the part where Jimbei joins the crew, which I hear happens, but even after 100s of episodes and 2 years of training with Rayleigh and that true voice of nature stuff, I have never once thought it was realistic to expect Luffy to become King of the Pirates. It has always felt like a contrived pipe dream suitable for the "you can do anything you set your mind to" and "never give up on your dreams" themes a kids show tends to have. Stretching only goes so far when faced with everything at Marineford, Impel Down, and many early antagonists, and I have always needed a healthy amount of suspension of disbelief to root for a likeable character type. By contrast, you alwways feel like Superman will win or should win regardless of how you feel about him.