Steward, Roach, Cus. Some of the best trainers, but they were never elite boxers. What makes a trainer a good trainer while they "fail"(fail compared to elites like Hagler or Lennox Lewis) as boxers. Or is it because they never had an elite trainer themselves that they "failed"?
Roger Mayweather, George Benton, Jack Blackburn. The fallacy of your argument is the major assumption you've made.
Commitment, personality, a stone cold knowledge of the basics, and the ability to teach it. The best boxers have a hard time transitioning to training because they advanced past the first layers of skill long ago, wrote their own book to feel comfortable in the ring, and have no idea how to teach a fighter who is 100% different from them in mentality and style. Steward is a great example; He's an amazing teacher.
Aye, - perhaps for some fighters, their skills are too instinctive to teach - they can't explain what to them comes naturally. I'd bet most of the good fighters who have succeeded at training were students of the art - analytical, tactical boxers rather than those with huge natural gifts (chin, athleticism).... I wonder if BHop will become a trainer? I see a parallel with the great football players who have failed at management. It's especially hard for those who start to manage at the lower levels - Roy Keane and Tony Adams wrote about their inability to understand why lower league players couldn't follow their teachings. Glenn Hoddle alienated players during training sessions by showing off skills he retained at age 40 that his squad couldn't hope to emulate at their peak.
It is very common in many sports for the guys who had to work hard and learn every detail, every advantage to turn into better coaches than the gifted athletes. Of course, that is a generality and there are many examples to the contrary.