I agree with your comment regarding the number of top anglers who have achieved success in other sports, in particular your view that dedication and focus is a common factor. The physical and mental co-ordination developed for success in one sport probably gives an advantage when learning to cast a fly line.
There would appear to be little in common with fly casting and my former sport of rowing! Casting a fly line demands only a modicum of physical strength but everything in mastering the technique. Conversely, rowing's image might be that of sheer physical strength. In fact the power has to be applied with correct technique to achieve speed and balance. Even in rowing, superior technique can prevail over raw strength.
I rowed at Cambridge University just after the war as the stroke of the highly successful 1947 Jesus Coll lightweight crew. With just 250 students and wartime rationing still in force, we didn't have the luxury of choice in creating a crew. However with excellent coaching, months and miles of training, the crew developed great cohesion and technique. We were never beaten and finished the season by winning the Grand Challenge Cup at Henley-on-Thames, giving away about 7Kgs a head to two champion European crews and Leander (an Oxford/Cambridge University combination.) A week later we won the eights at the Norwegian Championships in Bergin. Norway's rowing was strong, they won the Bronze Medal the following year in the 1948 Olympics Games, just feet behind Great Britain, Silver Medal, a crew I also stroked. This just shows that sheer size in rowing isn't everything.
Technique in casting is best exemplified by Mike Weddell, a former British and World Casting Champion, and by coincidence a talented athlete. Like all top sportsmen he makes casting look so easy, puting out consistent, accurate and, when required, long casts effortlessly.
Some years ago the Nelson fishing club were fortunate to arrange casting tuition for its members with Mike Weddell. With individual tuition and observing how Mike cast, my own casting was improved greatly. I'm confident that the mental and physical co-ordination developed in rowing helped a great deal to alter how I previously tried to cast. No, I don't get it right every time but am better able to recognize how and when things go wrong to make the necessary the changes to my technique.
Over the years, I've noticed how difficult it is for many experience anglers to change what they are doing, even with good coaching. While success in other sports is obviously not a pre-prerequisite to cast a fly line successfully, the hunger to be better and the discipline to focus must surely be an advantage.
Any well coordinated person, regardless of size, age or gender, if prepared to learn, can acquire the skills to successfully cast 30 to 60 feet, the distance at which the vast majority of trout are caught, and enjoy this wonderful sport.
Tight loops 'n lines!