The name given to triathlon is a bit deceiving. Yes, it’s comprised of three sports, but no, not equally.
For the sake of simplifying an argument, the sport seems to be 1 part swimming, 5 parts cycling, and 3 parts running. So, we might say that 5 out of 9 pro triathletes could go on to be pro cyclists given the proper amount of time to train for that (several years of specificity), 3 out of 9 pro triathletes could go on to be pro runners, and 1 out of 9 pro triathletes could go on to be pro swimmers. This is not true of course, as the world’s top runners are on a continent of their own, quite literally. However, if we exclude Africa from professional running I think the theory may hold up, or at least hold up quite a bit stronger.
I think part of the fact that this model doesn’t work stems from the fact that triathlon requires a high degree of athleticism to excel at, though it would require even more if the time required to finish each discipline was set up to be the same, i.e. an Ironman where top times in the swim were expected to take 2:40, top bike splits 2:40, and top run splits 2:40 (which is already about right for the run.). The sport that requires the most athleticism here is swimming. This is primarily due to the fact that water is approximately 78,400% more dense than air. Basically, you can run with horrible form and still hall balls, but if you don’t swim like a fish you’ll never be a top tier swimmer (I don’t think.) SOOOO, swimming is clearly the group of athletes to go after, BUT the problem is they are not blind, and can see that what they excel at best ALREADY only comprises about 11% of a triathlon, so the draw is much more limited.
I say if you really want to see triathlon draw more top caliber athletes, (pros that aren’t quite making the money spots in their respective sport because they are .01% too slow), you stop making triathlon a cycling sport with transitions from and to the start and finish and turn an Ironman into a 7 mile swim - 70 mile ride - keep the marathon.
For the time being I would say more professional swimmers are likely to convert to being a professional triathlete than any of the other combinations possible (swimmer → cyclist, swimmer → runner, cyclist → triathlete, cyclist → runner, cyclist → swimmer, runner → cyclist, runner → swimmer, runner → triathlete, triathlete → cyclist, triathlete → swimmer, triathlete → runner)